So you're seeing the firewall icon in LW (brick wall)?
Download a file from here & see what speeds you get
http://www.magnetmix.com/video.shtml

Has anything at all changed since your speeds have dropped (new ISP or plan, new hardware etc)?

What are your details (excuse any repeat questions)?

OS(Win XP etc)
LW & Java version (help > about LW & you'll see)
Firewall you're using
Modem/router brand & model
ISP
Connection type (DSL etc)
Where you connect from (home, school, student accommodation etc)
Are you sharing a connection with other computers
Is this your first try with LW
Have you tried other p2p apps & if so, do you have the same problem