FWIW enabling this did not give me any noticeable ui speed increase. It did however cause limewire's ui to act very strangely, in that random areas of the ui (particulalry active swing components, like a menu) would have odd blocks of colored lines and minor distortions o fthis nature. Not that this has anything to do with limewire, just thought I'd let you know.
My specs are as follows:

limewire 4.2.5 on slackware 10.0, video card is nvidia TNT2 32mb

burn@nonteef:~$ java -version
java version "1.5.0-rc"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-rc-b63)
Java HotSpot(TM) Client VM (build 1.5.0-rc-b63, mixed mode, sharing)

burn@nonteef:~$ X -version

Release Date: 18 December 2003
X Protocol Version 11, Revision 0, Release 6.7
Build Operating System: Linux 2.4.26 i686 [ELF]

burn@nonteef:~$ uname -a
Linux nonteef 2.4.26 #6 Mon Jun 14 19:07:27 PDT 2004 i686 unknown unknown GNU/Linux

--ps
forgot to mention that i am using the nvidia driver, tho it may be a bit out of date. I'm not sure how to check the version - in the meantime here is an excerpt from my xorg log:
(II) NVIDIA(0): NVIDIA GPU detected as: RIVA TNT2 Model 64/Model 64 Pro