I hate to suggest this, but looks like a clean install is the last option. You should be getting far better speeds.

Before searching your hard drive and deleting all LimeWire files, there is another option.

Create a new user in System Preferences, Accounts (with Admin Privileges), and install LW there. It will be just like installing on a almost new hard drive. Try the download speeds from that account. If there is no difference, the problem is either in your system or upstream of you.

(Once done, you can log back into your usual account and delete the one you had just created.)