Which version of Java do you have? Are you using LW 5.1.1 or 5.1.2?

Try updating your Java. There's a link in the thread below. Download the JRE offline installer & remove any other Java versions installed (via Add/Remove Programs) before installing the new version

Also see the thread below for other tips - purging the download queue & deleting LW's preferences folder could help as well. If you're downloading lots of files at once, try just downloading 1 or 2 at a time.

http://www.gnutellaforums.com/333278-post2.html

If you're still having problems after that, in LW go tools > options > advanced > super really advanced > performance. Disable ultrapeer, DHT & TLS > ok.

If you decide that you'd like to revert to LW 4.18.8, check the link below for info.
http://www.gnutellaforums.com/341266-post3.html