I have to admit that it's difficult to fully understand the various combinations of devices, wired vs. wireless, secure vs. insecure, etc. But it needs to be understood that in some cases, differences can be expected when comparing strictly based on wired vs. wireless.
Wire is FULL DUPLEX, whereas wireless is typical HALF DUPLEX (putting aside MIMO for the moment, which in my experience, rarely does anyone have it fully supported). A wired connection can both send and receive data at the same time, whereas a wireless connection can NOT. Wireless can only send or receive at any given time, and when doing so, *all* other wireless connections using the same freq/channel must wait! That's what makes a situation like having the PC act as a middleman between two FTP servers, both connected over wireless, a worse case scenario. And why it's generally a bad idea to have a wireless device acting as any kind of NAS. You're creating a situation ripe for poor performance, at least when compared to wire.
There's also the issue of the PC itself (or more specifically, the application) not being particularly efficient when moving data between servers, particularly if single threaded and having to wait for IO operations to complete. Sometimes applications just don't handle it well, and why you can't necessarily blame it all on the link layer (wired vs. wireless).
Finally, throw encryption into the mix, and things can get even more unpredictable since not all platforms handle it equally well either.
That's why it's tricky when it comes to this kind of testing. It's not always obvious how even subtle differences can have a large impact. And the more variables you introduce, the trickier it gets.
All that said, if you're seeing exceptional differences in performance between wireless and wired, there may indeed be some other issue (like the app). But I just want to be clear that wireless is inherently far less efficient compared to wire when dealing w/ large data transfers. This is a case where MIMO might make a difference, but as I suggested, many clients devices don't support it, or have compatibility issues even if they do.
