Why are transfer speeds so slow from USB3 SD card to PC?
Solution 1:
Problem could be "all of the above", with the more likely candidates being a low quality USB card or (a distant second) possibly a bad USB driver. Does the SD Card have a number on it ?
The multiple files are unlikely to be the issue if coming from SSD.
It could also be a faulty cable, faulty reader. But the SD card speed is the most likely candidate - SD cards are, as a rule, not that fast (but there are exceptions).