Check with the devs at LimeWire Forums

IIRC, LW has implemented in internal throttle, to prevent LW from hogging the bandwidth. That may be what you are seeing, but check with the devs in case this is a bug.

Given your results and more than adequate bandwidth, you should be able to download faster, so the devs should be able to let you know if the throttle is within LW, or whether you need to look elsewhere.