Only whilst LW is running. After it is quit is should be fine. But yes, LW is a top heavy program & apple's java is not especially efficient (not as good as Sun's Java.) Apple's java has memory leaks, & doesn't free up memory when it should. (So it is also partly apple's fault.)

* When LW becomes slugglish, closing & re-opening will refresh it.
* Keep shares on OSX to below 1,500.
* Try to keep incomplete files to a lowish number; Tip: Purge the Downloads Queue! (click on blue link).
* Disable Ultrapeer capabilities for a start. Go to Tools > Options > Advanced > Performance & disable UP.
* Set both uplds & downlds to autoclear.
* Close all LW windows you don't need open at that time. eg: Connections window.
* If you're not in front of your comp, then hide LW in the dock.
* Don't use the Resume button on all incomplete files. This can have side-fx such as affect LW's connection quality. May queue your files for much much longer. This will also stress LW & your comp. If you use Resume/Find sources then just apply it to a couple or so at one time.
* Better is to use the manual search function to re-search the file. Also control-click the search result -> Advanced & choose search again or Find More Results.