TLS is used to encrypt your connections so ISPs will not be able to know what it is that you are doing. So DEFINITELY, keep it activated.

Note: Developers dedicate a long time to implement improvements to software. So please, do not just deactivate because it "Uses more of your CPU". Personally, I use gtk-gnutella (it is similar to LimeWire but written in C not in Java). It uses only 1%-2% of my CPU even with TLS enabled.

Newer CPUs have been improved to encrypt faster than older CPUs, so if LimeWire uses a lot of your CPU, it is probably because your CPU is old.