I'm not the developer so they can shed more accurate light on this but from what I know:
1) LiteSpeed does not employ threads.
2) By default PHP fcgi childs are handled by PHP's internal fcgi pool mech, not lsws. Lsws just open up and estabilish communication with php fcgi handler..
Which platform are you using lsws on? Linux/FreeBSD/Solaris? What version of kernel? Which verson of LiteSpeed are you runing? Which version of PHP? Are you using any special PHP extensions not bundled with PHP such as turck-mmcache? These will help debug this problem for ya.
Are you using Lsws bundled php package or a self compiled one? Output of phpinfo would be good to reveal any conflicts.
Linux is very aggressive at used memory so if you have program that might use 5MB out of 2G sysstem, after a few days/hours of aggressive use of the HD, free memory will be 0 and total used memory will be 2G. This is because linux's buffer cache mech is extremely agressive and will use all available idle ram to cache file i/o.
Hope everything works out.