There is a high possibility Insight is shaping your p2p bandwidth. Many ISP's use a special program that detects p2p use & shapes that bandwidth, whilst giving priority to surfing. You'll find Insight on this list
ISP's known to BLOCK or Filter p2p or specifically Limewire. So I'd suggest you're lucky you can still use LW with only minimal problems. Normally download speeds depend upon your sources, but in some cases the ISP is at fault. Keep trying to get more sources for your downlds by keep searching & using the Get more Results option (ie: right-click search results tab ...)