It's difficult to know why. It could be due to a no. of reasons.

Sometimes closing LW & re-opening will allow you to connect thru different peers & thus get better speeds.

Else, try deleting your LW Preferences folder whilst LW is closed; instructions here: Fixes for Limewire

Try downlding a magnet link file from here to see what speeds you get: http://www.magnetmix.com/video.shtml

BTW did you look at the Sticky at the top of this section.