Try these things:

1. Steps 1 AND 3 here Fixes for Limewire. ie: use your add/remove control panel to remove Java & then reinstall using only the "Offline" version, http://www.java.com/en/download/manual.jsp

2. Tip: Purge the Downloads Queue!

3. If you have iTunes sharing enabled, then disable it. LW menu bar, Tools>Options>iTunes>Sharing.

4. Don't share much more than 2,000 files.

5. Defragment your Hard Disk drive. p2p'ing fragments the HDD very quickly which can slow your system down.