The easy/quick solution to this problem is add more ram and cpu power.

I've used LimeWire on 3 different machine configs, and found that my celeron 600 Mhz/512 MB were quite enough for a 6 MBit U/D wire having 24 nets running.

CPU usage weren't much over 80 %, on my dual P3 933 MHZ 512 MB you'd bare notice LimeWire was running, and on my Celeron 1100 Mhz/256 MB cpu usage hovers at about 10-20 %

The amount of ram LimeWire uses, I would suggest, depends on the number of files you are sharing and LimeWires Java VM