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.
There are four SD card formats:
UHS Speed Class
Application Performance Class
Video Speed Class
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.
|Workload||Min Write Speed||Rating|
|8K video @ 60/120 FPS||60MB/s||V90 or V60|
|1080p/4K video @ 60/120 FPS||30MB/s||V30, U3|
|720p/1808p/4K video||10MB/s||Class 10, V10, U1|
|720p/1080p video, 4K video possible||6MB/s||V6, Class 6|
This info from https://www.makeuseof.com/tag/best-fastest-microsd-cards/