Have you seen this thread (sorry if you have)? There are some good tips: A few things to consider while configuring Limewire for optimal performance...

Do a speed test here: Speedtest.net - The Global Broadband Speed Test
When you do the test, don't have any other programs running... just the page with the speed test open

Answer as many of these questions as you can: System details - help us to help you -> Provide these details when you post!!!

Let us know if you're stuck with any of those questions