The protocol is riddled with latency issues. These can eat CPU without using much network bandwidth. With Ethernet, the situation is improved, but there's still latency in certain places.

Also, did you have tracing turned on? It'd have warned you at startup if you had, so you probably didn't. If you did, there's a shedload of data going to disk, and that can introduce a bunch of CPU usage -- every line needs to be sprintf'ed, which eats CPU.