I have noticed several patterns in this behavior.

1. Limewire cpu use spiking often occurs right after a download is finished. As if the occasion calls for major number crunching, even if the "verifying" and "saving" stages flash by quickly. After a few seconds LW slows to a crawl for a minute or two.
2. Performance on my machine seems to degrade drastically if LW's memory use climbs above about 100M. If it's 80 it's very responsive. If it's 100 it's unbearably slow. I'd suspect thrashing except that the machine has over a gig physical ram FREE, NOT IN USE when this happens! With 2GB in the box performance should degrade much more gracefully, and only when total mem use is a LOT more than just ~6-700M total commit charge. Who designed this piece of ****?
3. What the FU&! is it doing that can saturate a 3GHz dual-core CPU? Nothing an event-driven application ought to be doing, methinks. And at a priority equaling the event-handling for the UI, at that, rather than at a much lower one.