The "To Keep Up" Wiki

info about different SD and Micro SD cards and their ratings Class 10 vs 6 vs 4; UHS-I and UHS-II; V60 and V90; and more.

Many cheap too-good-to-be-true SD cards are indeed not the rated size. See this article in MakeUseOf to spot fake ones. It discusses software you can use to see if the SD or TF card is not the size it claims to be: FakeFlashTest, H2testw, ChipGenius. What to do if you're stuck with a fake card (maybe you can determine its real size and use it or maybe you can fix it using DiskPart). All software is Windows.

From https://www.makeuseof.com/tag/5-mistakes-avoid-buying-next-microsd-card/

There are four SD card formats:

1. microSD: Has a capacity up to 2GB, and works in any microSD slot.
2. microSDHC: Has a capacity of more than 2GB and up to 32GB, and works in hardware that supports either SDHC and SDXC.
3. microSDXC: Has a capacity of more than 32GB and up to 2TB (although at the time of writing, 1TB is the largest available card), and is only supported in SDXC-compatible devices.
4. microSDUC: Supports cards up to 128TB, and will require a compatible device.

Speed Class

• Class 2: At least 2MBps.
• Class 4: At least 4MBps.
• Class 6: At least 6MBps.
• Class 10: At least 10MBps.

UHS Speed Class

• U1: A write speed of at least 10MBps.
• U3: A write speed of at least 30MBps.

Application Performance Class

• A1: Minimum random read speed of 1500IOPS; random write speed of 500IOPS.
• A2: Minimum random read speed of 4000IOPS; random write speed of 200IOPS.

Video Speed Class

• V6: Minimum write speed of 6MBps.
• V10: Minimum write speed of 10MBps.
• V30: Minimum write speed of 30MBps.
• V60: Minimum write speed of 60MBps.
• V90: Minimum write speed of 90MBps.

This diagram comes from MakeUseOf at the URL above.
The SD Card Association recommends U3 or better for 4K video. If your card's write speed is too slow, it will drop frames and produce stuttering video.

This info from https://www.makeuseof.com/tag/best-fastest-microsd-cards/