MicroSD Express vs. The Legacy Bus (UHS-II, UHS-III)

The market for small, fast removable storage is splitting. This analysis shows MicroSD Express is not just another step for SD cards. It is a complete replacement. It divides the market by swapping the old SD bus, which ended with UHS-II and UHS-III, for the faster PCI Express (PCIe) and NVMe interface used in modern computers.

Key Findings at a Glance

• Performance: The two standards are built for different jobs. MicroSD Express gives SSD-like performance. It focuses on high burst speeds and a large increase in random read/write (IOPS) speed. This makes it a better standard for running applications, like games, by cutting load times. On the other hand, the UHS-II standard is still the measure for guaranteed minimum sustained write speed (like V90). This is the main number for high-bitrate professional video recording.
• Compatibility: The standards do not work together at high speed. The central and widely misunderstood point is that a MicroSD Express card in a UHS-II slot will not run at UHS-II speeds. It will fall back to the slowest shared speed: the UHS-I bus. This limits performance to a 104 MB/s maximum.
• Market & "UHS3": The standards serve two separate markets. UHS-II is common in the professional digital camera industry. MicroSD Express adoption, however, is pushed almost entirely by handheld game consoles, mainly the Nintendo Switch 2. The "uhs3" (UHS-III) standard is a "paper standard" that failed commercially. It saw no market use.

The choice between these standards now depends entirely on the host device. They are no longer direct competitors. They are specialized tools for different tasks.

Which Card is Right for You?

Performance at a Glance: Charts

The performance gap is not small. It is a completely different class of speed. MicroSD Express uses NVMe, the same protocol as internal computer SSDs, giving it a massive lead in random I/O operations (IOPS). This is the speed that makes games and apps load quickly.

Architecture: Why The Pins Are Different

A common point of confusion is that both UHS-II and MicroSD Express use a "second row of pins." The pins are not the same. They are electrically different and built for entirely separate interfaces. A MicroSD Express card adds pins for PCIe. A UHS-II card adds pins for LVDS. They cannot speak to each other, which is why they both fall back to the only interface they share: the front row of UHS-I pins.

Foundational Architecture: PCIe/NVMe vs. The Legacy SD Bus

The performance gap between MicroSD Express and UHS-II comes from two completely different technologies.

A. The MicroSD Express Change (SD 7.0, 7.1, 8.0)

MicroSD Express, part of the SD 7.1 specification, is a complete architectural break. It drops the traditional SD bus interface. It instead integrates two technologies from high-performance computing:

1. PCI Express (PCIe) Interface: This is the same high-speed serial bus used on a computer's motherboard for graphics cards.
2. NVMe (Non-Volatile Memory Express) Protocol: This is a low-latency communication protocol made for accessing modern flash storage.

This combination effectively turns the microSD card into a "miniature SSD." The use of NVMe is especially important. It is designed for high parallelism and low latency. This allows a huge leap in random read/write operations (IOPS), something the old SD bus was never designed to handle.

B. The UHS-II & UHS-III Standards

UHS-II (Ultra High Speed-II) was the previous major leap in SD card performance. Its main update was also adding a second row of pins. However, its underlying technology is entirely different from Express.

Its core technology is not PCIe. Its second row of pins uses Low Voltage Differential Signaling (LVDS). This is a high-speed data transfer method that allows a theoretical bus speed of 312 MB/s. This 3x speed increase over the 104 MB/s limit of UHS-I made UHS-II the standard for high-end video recording.

UHS-III (Ultra High Speed-III) was announced by the SD Association in 2017. It was a minor update that used the exact same physical pin layout as UHS-II but doubled the speed to 624 MB/s.

Despite its specifications, UHS-III is a "paper standard" with zero commercial adoption. It failed because UHS-II was already fast enough (312 MB/s) for most 4K video. Devices needing more speed moved to CFexpress. The gaming market needed better random I/O, which UHS-III did not address. SD Express was announced shortly after, making UHS-III obsolete.

Performance Benchmarked: Throughput, Latency, and Endurance

The different architectures show in their real-world performance numbers, which are built for their target applications.

A. Peak Sequential Throughput (Read/Write)

This number represents the maximum burst speed of the card, such as transferring one large file.

• MicroSD Express (SD 7.1): Real-world products (PCIe 3.0 x1) are benchmarked achieving ~800-880 MB/s read and ~600-700 MB/s write speeds.
• UHS-II: The 312 MB/s theoretical bus speed is nearly achievable. High-end V90-rated cards benchmark at approximately ~304 MB/s read and ~234 MB/s write.
• UHS-III: Theoretical 624 MB/s. No commercial products are available to benchmark.

B. Sustained Write Performance (The Videographer's Metric)

This is the minimum guaranteed write speed, the most important number for professional video recording. If a card's write speed dips below the camera's video bitrate, it causes "frame dropping" and corrupt footage.

UHS-II (The V90 Standard): The UHS bus is linked to the Video Speed Class (V-Class). V90 (90 MB/s) is the gold standard for 8K or high-frame-rate 4K/6K video. A V90 rating is only possible on a card with a UHS-II or UHS-III interface. This is a limitation of the bus. The UHS-I bus has a theoretical maximum of 104 MB/s. It is impossible to guarantee a 90 MB/s minimum write speed (V90) on a bus that maxes out at 104 MB/s. There is no performance overhead.

MicroSD Express (The E-Class Standard): Express introduces new SD Express Speed Classes (E150, E300, etc.), which also guarantee a minimum write speed in MB/s.

Benchmarks have revealed a paradox: a high-end UHS-II V90 card can show better sustained write performance than an early MicroSD Express card. This is explained by their design. The Express card acts like a consumer SSD. It uses a small, fast cache for high burst speeds. Once that cache is full (like during a long video recording), performance can drop as the card writes to slower NAND flash. The UHS-II V90 card is built specifically for video. Its controller is made to never drop below 90 MB/s, sacrificing peak burst speed for steady endurance.

C. The NVMe Advantage: Random Read/Write (IOPS)

This is the biggest performance difference and the entire reason for MicroSD Express's existence. Random I/O (Input/Output Operations Per Second) measures how quickly the card can read and write many small files. This is the main workload for running an OS or loading a video game.

• Legacy (UHS-II): UHS cards use the "Application Performance Class" (A1 or A2). The best A2 standard requires a minimum of 4000 Read IOPS and 2000 Write IOPS. This is a bottleneck for game loading.
• MicroSD Express: By using the NVMe protocol, the Express card is designed for thousands of parallel requests, just like a PC SSD. A typical card is rated for 65,000 Read IOPS and 52,000 Write IOPS.

This is a 16-fold increase in random reads and a 26-fold increase in random writes over the best A2-rated UHS-II card. This is a large difference for gaming. It allows for reduced loading times and smoother texture streaming.

Performance Specification Matrix

The "UHS-I Trap": The Compatibility Problem

The most expensive mistake a consumer or professional can make is assuming cross-compatibility. A MicroSD Express card is NOT backward compatible with the UHS-II or UHS-III bus.

When a MicroSD Express card is inserted into any non-Express slot (UHS-I, II, or III), the host device cannot establish a PCIe link. The only shared electrical interface is the first row of pins. This first row is defined by the UHS-I bus. Therefore, the card and host default to UHS-I mode. This limits performance to a maximum of 104 MB/s. This creates a "compatibility dead end."

Quick Guide: A Visual Decision Tree

The compatibility issue makes choosing a card difficult. This flowchart simplifies the decision based on your specific host device.

Practical Hurdles: Power and Thermals

This heat problem is so severe that actively cooled MicroSD Express readers (with fans) now exist. This is a clear sign that heat is a primary performance bottleneck. Host devices must now be designed to manage the card slot's heat.

Behind the Slot: Host Device Design Choices

Why would a device maker choose one standard over another? The decision involves cost, power, and physical design.

A. Controller and Licensing Costs

Integrating a card slot is not free. The host device needs a "controller" chip to communicate with the card.

• UHS-II Controller: This is a mature, well-understood piece of technology. Its licensing is handled through the SD Association. For camera makers, this is a known, reliable cost.
• MicroSD Express Controller: This is far more complex. The device must have a spare PCIe lane available from its main processor (SoC). This is common in PCs but not in cameras. Furthermore, the device maker must license both the SD Association standards and the (more expensive) PCIe/NVMe standards. This adds to the bill of materials.

B. Power and Thermal Budget

As mentioned, an Express card can draw over double the power of a UHS-II card. This is a critical problem for a device like a mirrorless camera, which runs on a small battery. The extra power draw would shorten battery life. The extra 1-2 watts of heat would need to be dissipated, potentially forcing the camera body to be larger or require internal heat sinks, which adds cost and weight.

A handheld console like the Switch 2, which is designed with a larger battery and active cooling (a fan) for its main processor, is already built to handle this type of thermal load. Adding a hot-running MicroSD Express slot is a smaller engineering challenge for that device.

Market Analysis: Adoption, Cost, and Future

The technical differences have created two separate markets.

A. Device Adoption

• UHS-II: The Professional's Standard: This is the standard for modern pro mirrorless and DSLR cameras. Manufacturers like Sony, Nikon, and Canon use UHS-II slots to support V60 and V90 video recording.
• MicroSD Express: The Gamer's Standard: The consumer market for MicroSD Express is currently being created by a single product line: the Nintendo Switch 2. This single console provides the large sales volume needed to justify production. Adoption in cameras and mainstream laptops is, as of late 2025, almost non-existent.

These trends show the "great divergence." The camera market has no reason to adopt Express. It would require a costly thermal and power redesign. Its main benefit (high IOPS) is useless for video. Its main weakness (potentially poor sustained writes) is a deal-breaker. The gaming market cannot use UHS-II, as its random I/O is a performance bottleneck.

B. Cost-Benefit Analysis

The pricing of these two standards shows their different target markets.

The large price difference does not mean V90 cards are a "rip-off." It shows their niche. MicroSD Express uses high-volume NVMe controller and 3D NAND technology from the PC SSD market. It benefits from large economies of scale.

A UHS-II V90 card is a low-volume, specialty tool. The high price reflects the R&D and manufacturing cost to create and validate a controller that can guarantee a minimum of 90 MB/s sustained write performance for hours in a tiny form factor without throttling. Consumers are paying for the reliability of the V90 guarantee, not just the burst speed of the UHS-II bus.

Future Roadmap: SD 8.0, SD 9.0, and Market Consolidation

The split between standards is likely to continue. The SD Association has already defined the next steps for the Express standard.

SD 8.0 Specification (PCIe 4.0)

The SD 8.0 specification, announced in 2020, updates the Express interface in two key ways:

1. PCIe 4.0 Support: It upgrades the bus from PCIe 3.0 to PCIe 4.0, doubling the theoretical single-lane speed to ~1970 MB/s.
2. Second PCIe Lane (x2): It adds an option for a second PCIe lane, doubling the bandwidth again to a theoretical maximum of ~3940 MB/s.

As of late 2025, no commercial MicroSD Express cards using the SD 8.0 specification are available. The market is still adopting the first-generation SD 7.1 (PCIe 3.0 x1) cards. The SD 8.0 standard is aimed at full-size SD Express cards first, which can more easily handle the heat and power of a dual-lane PCIe 4.0 connection.

SD 9.0 and Beyond

The SD 9.0 specification, announced in 2023, focuses on firmware and security features rather than speed. It adds new "boot support" and "trusted computing" features. This strongly suggests the SD Association sees the future of Express cards as more than storage. They are positioning them as bootable mini-SSDs for running secure operating systems on industrial or mobile devices.

Meanwhile, the UHS-II bus has no public roadmap for a "UHS-IV." Its development has ended. The professional video market that needs more speed than UHS-II has already moved on to the CFexpress Type A and Type B formats, which are also based on PCIe/NVMe but in a larger, more heat-resistant shell.

Visualizing Dataflow: A Canvas Demonstration

To understand the difference in data handling, this simple canvas visualization shows the data flow. The legacy SD Bus (UHS-II) acts like a single, wide highway. The new Express (PCIe/NVMe) standard acts like a massive network of smaller, faster paths, which is why it excels at random I/O (handling many small files at once).