High CPU usage has always been a problem with Gnotella for me too. For what it's worth uploads and downloads don't seem to affect it, but the more network connections you have the higher the CPU usage goes. Even just four connections eats up all my CPU and cause the program to freeze for several seconds at a time.

I remember this was a common problem in several early clients, but most have this solved now. I hope it's fixed soon in Gnotella too - really I think it's the only thing holding it back from being the absolute best Gnutella client around.