Do you have lots of incomplete downloads or lots of shared files? The next LW 5.3 will handle such better, which should be out within weeks at a guess.
Are you downloading gnutella network files or torrent files?
You might like to try a couple of things: Go to LW's menu bar, Tools -> Options -> Advanced -> SuperReallyAdvanced -> Performance, & disable Ultrapeer ... and possibly TLS.
Try the LW 5.2 beta instead:
LimeWire 5.2.4 (Beta) - Lime Wire
If still problems close LW, go to the LW preferences folder, remove the Downloads.dat file. Then delete the LW preferences folder. Open LW, then close it again. Replace the old Downloads.dat file back into the preferences folder. Open LW again. (Explained in more detail in a warning part of first post here just before point #5 here:
Fixes for Limewire (click on blue link))