C10, U3, V30, A2 and microSD Express ratings reveal sustained performance and compatibility that headline read speeds often conceal.
What looks like a string of cryptic markings on a microSD card is often the difference between a card that merely stores files and one that performs reliably under load. The large number printed on the box, such as 190 MB/s or 200 MB/s, usually refers to peak read speed, which matters when copying photos and videos to a computer. It says much less about how well the card can sustain writes during long video recordings, game installs or app use, where the minimum guaranteed write performance and the device’s own requirements matter far more, according to guides that explain SD card classes and real-world use cases.
That is why labels such as C10, U3 and V30 deserve more attention than the headline figure on the packaging. These markings describe minimum sequential write performance, not marketing peaks. C10 means at least 10 MB/s, while U3 raises that floor to 30 MB/s. The Video Speed Class was created to make that distinction clearer for recording workflows, so a V30 card guarantees at least 30 MB/s of sustained write speed under the standard’s conditions. Several technical guides point out that these labels can appear together on the same card because they belong to different classification systems, not because they repeat the same promise.
The interface label is another source of confusion. UHS-I, UHS-II and UHS-III refer to the bus between the card and the host device, not the minimum write speed itself. In theory, the classes top out at 104 MB/s, 312 MB/s and 624 MB/s respectively, but those are interface ceilings rather than guarantees of real-world throughput. A card may be capable of more than a basic reader or older phone can use, while a fast-looking card can still be bottlenecked by a weaker device. The practical advice found in product guides is consistent: the card and the reader must both support the same mode if buyers want the advertised speed to be meaningful.
For apps and mobile devices, A1 and A2 are the labels that matter most. They describe random performance across many small files, usually measured in IOPS, which is more relevant to Android app loading and system responsiveness than to a single sequential video file. The commonly cited minimums are 1,500 read IOPS and 500 write IOPS for A1, rising to 4,000 read IOPS and 2,000 write IOPS for A2, alongside the standard sequential floor. However, some guides note that A2 performs best only when the device supports features such as command queuing and caching; without that support, the advantage may be limited.
The newest category is microSD Express, which uses a different interface closer in concept to SSD technology than to older microSD designs. Its E150, E300, E450 and E600 ratings indicate minimum performance tiers of 150, 300, 450 and 600 MB/s under the standard’s conditions. This matters because newer hardware may require that format specifically. Guides discussing current use cases note that the first Nintendo Switch uses conventional microSD cards, while Nintendo Switch 2 requires microSD Express for storage expansion, making compatibility just as important as speed. For 4K video cameras, action cams and gaming devices, the safest approach is still to start with the device manual, then match the required class rather than chasing the biggest number on the box.
Disclaimer: This content is intended for informational purposes only. Readers are advised to exercise their own judgement, conduct due diligence, or consult a qualified expert before acting on any information provided.
