Well, I think it's to do with the interaction of AMD cpu (Duron), the particular type of video card/driver (KLE chipset with VIA tech VT8361/8601 graphics controller, Trident 3D display driver) on the PC and Java (1.5x?).
I have had problems with this "slowness" with both LimeWire and XNap on the same machine (not at the same time of course). The funny thing is I use Firefox as a browser on that machine, which makes use of the same Java, but no problems with slowdown.
Also, I have another PC with an AMD Duron processor but different graphics chipset etc. running Wi*** & Java 1.5x and absolutely NO problems. Which makes me think it's to do with the graphics chipset/controller.display driver. I'd be interested in these configurations with other peoples PC that have the slowness problem.