With LimeWire 4.2.6 I was having a lot of trouble doing anything with my computer. CPU usage was generally around 80%.

I decided to buy LimeWire Pro. My CPU usage is half what it used to by, I'm connected to more Ultrapeers, searches are faster, and downloads are much quicker. Oh, and it doesn't crash like 4.2.6 did, either.

I've got a sneaking suspicion that the free version of Limewire actively throttles itself back. Sometimes when connecting I'd reach turbocharged connectivity, only to have the program drop back out of that level of connectivity. If Limewire actively limited itself in the free version, it could account for the extra CPU use we see.