This has always been an issue on mac due to apple using their own version of Java which is inferior to the java used for other systems.
The problem is Java has problems releasing memory. So the Virtual Memory (VM) builds up over time. The longer LW is on, the more VM is used and eventually starts to slow the system down.

Workarounds:

* Make sure your system drive has plenty of space.
* Close LW periodically which releases the VM used up. BTW you can check how much VM has been used via the Activity Monitor found in the Utilities folder. Keep an eye on it over a period and you will see what I mean, VM builds up gradually the longer LW is used.
* Share less than 2,000 files, or if videos, share even less. The more files shared, the greater VM will be.
* Reduce incomplete files, Tip: Purge the Downloads Queue! (click on blue link) because the more incomplete files you have, the greater VM will be.
* Keep all LW windows closed except those you need at the time. eg: Connections window.
* Disable Ultrapeer capabilities for a start. Go to Preferences Options > Advanced > Performance & disable UP. In fact, disable all options on that window as they all add to VM when enabled. Quick access to preferences is Command-Comma keys; ⌘ ,
* Set both uplds & downlds to autoclear under preferences.
* Hiding LW in the Dock may also help to reduce some ram and vm use.
* Close off old search tabs.
* Also 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.