If it's over SMB/Windows file sharing then you might be looking at some kind of latency-induced limit. AFAIK SMB doesn't stream uploads, they occur as a sequence of individual write operations, which I'm going to guess also produce an acknowledgement from the other end. It's possible something like this (say, client waiting for an ACK before issuing a new pending IO) is responsible

What does iperf say about your client/server combination? If it's capping out at the same level then networking, else something somewhere else in the stack.

I noticed recently that OS X file IO performance is absolute garbage because of all the extra protection functionality they've been piling into newer versions. No idea how any of it works, all I know is some background process burns CPU just from simple operations like recursively listing directories