This part of a tutorial I wrote for LCDs should help:
Enabling the bytecode interpreter
Essentially, you need to enable the bytecode interpreter in FreeType (the font rendering library) to get your fonts to look as good as Windows or Mac OS X (due to patent issues, from FreeType 2 this is disabled by default - you need to renable it, then rebuild FreeType and overwrite the FreeType libraries that ship with Slackware's X.Org).