[geerlingguy]

As @written says, there is a huge limitation on the microSD card reader on the Pi itself; but also, in my testing on a UHS-II USB 3.0 card reader on my MacBook Pro, most of the cards can't sustain more than 30-40 MB/s write even if the specs say they can.

Large block read speeds are usually pretty accurate, but manufacturers take quite a bit of liberty with their performance claims. And random I/O is pretty terrible in almost every case.

Remember that these types of cards are _usually_ optimized for large file I/O since they're used in dashcams, GoPros, and the like—use cases that are vastly different from a general computing device running Linux!