[razemio]

I am uncertain if USB can be a bottleneck there, but if I am not mistaken, SSDs offer better (more?) cache. This dramatically reduces time needed for non-sequential read/write operations.

[rmykhajliw]

USB - 30Mbyte/sec theoretically for RPI, but in reality somewhere around 20Mbyte/sec

SD cards - class 10 tested on RPI show the same performance 21-27Mbyte/sec

So the issue mostly in throughput performance of the interface