Not OP, but in my experience it's the broad range of SD card quality on the market, and the sketchy/'temporary' physical connection between the gold contacts on

Not OP, but in my experience it's the broad range of SD card quality on the market, and the sketchy/'temporary' physical connection between the gold contacts on the card and the gold contacts in the card reader. In this case, the reliability issues result in the storage device dropping out sporadically entirely during reads and/or writes, or dramatically varying speeds sometimes as low as several KB/s.