There are a few things that make LimeWire slow and that can be improved. However I don't think anybody has seriously started working on it:

http://www.limewire.org/project/www/performance.htm

It's a document Christopher Rohrs wrote before leaving LimeWire. - I was quite surprised to see, that he left, - he suddenly disappeared from the LimeWire.com site. That means there are only three developers left, working on LimeWire. I think it's definitely time, more open-source developers start working on LimeWire.