The Pi is 10/100 Ethernet. 100 Ethernet is 12.5 MB/s. This is almost always the first bottleneck you hit.
If the Pi had gigabit E, then you'd be looking at 125 MB/s, and the bottleneck would then likely be USB. If it's USB 2 you're looking at an effective speed limit of 35 MB/s (with USB 3 you're looking at around 400 MB/s).