Often, your speeds will drop substantially on transferring numerous smaller file sizes, and, certainly if reading from/writing to a slow spinning drive even more so. Certainly you should not expect advertised drive or USB3/USB3.1 speeds constantly. (Even if transferring from SSD to SSD)
Your best (reasonably priced) bet at sustaining highest possible USB transfer speeds (both read and write) would probably be with one of the USB3/NVME adapters using one of Intel's 660P drives. (This assumes your file source is not a bottleneck, but, any spinning hard drive source is going to be lucky with sustained ~100-150 MB sec transfers, which will fall to 10-30 MB/sec when doing smaller files)