Second this. Just today, I was moving a couple terabytes from my work PC to my media server at gigabit speeds, and the transfer was absolutely hammering the poor quad core i5 I’ve got in it. Surfing the web was less than pleasant for an a hour to two there.

Edit: I didn’t see OP’s reply to this comment when I first wrote it. I agree decompression is the most likely culprit, as it can be CPU intensive and compression ratios vary quite a lot from game to game.