Download speed depends on your connection & the connection of the host(s) you're downloading from, how well you've both got your LW set up, whether it's "peak time" for your ISP & there's congestion etc etc. If speeds have suddenly dropped from all types of sources then I'd be a bit suspicious... who is your ISP? Download a file from here & see what speeds you get
http://www.magnetmix.com/video.shtml

There's a lot of good info here for optimising LW's performance
Configuring LW for best performance

You could also try deleting your LW preferences folder (when LW's closed). Instructions here
Fixes for LimeWire

How's your speed when you're just using the internet? You can do a speed test here
http://www.dslreports.com/speedtest?more=1