It's just what I've experienced myself when copying a lot of files. E.g. "add 50k photos from network drive to Photos" or "perform initial Time Machine backup". Over cable I'd say it's at least twice as fast, haven't done any measurements though.
> Over cable I'd say it's at least twice as fast
That could be be because of the bandwidth difference, if wireless isn't 802.11ac (or if the cable isn't 100Mb/s).
Is it also the case that copying a single file of comparable size doesn't show the same speedup?