This might sound like a silly suggestion but scan your computer for trojans/spyware/ad ware ot malware. I was having an issue with slow downloads and after double checking my connection with my ISP then making sure everything was fine with limewire i did a scan..low and behold i found two hijackers plus a few other nasty little trojans/spyware on my computer.
got rid of them now the only slow downloads i get are from those on modem connections.

The trojans/spyware did not come from Limewire, the came from other sources on the net.