It uses 100% of the CPU and grinds the disk for a while at startup as it's scanning your shared files and getting the bitrate of MP3s. It should go down after it's done with that. If you have a large number of files shared, the CPU usage could jump up fairly high as incoming searches are porcessed.

Hopefully the LimeWire developers will start writing a database of your shared files to the disk in new versions or LW so it doesn't have to scan them all at every startup. And I hope they figure out a way to reduce the CPU usage for people with large libraries by either doing whole word searches instead of partial matches or maybe using native C code for the search word lookup.