I rooted my Galaxy S (first one) so that I could install custom firmwares and lag fixes as out of the box 2.1 and 2.2 were awful. After upgrading to 2.3 I have not had a reason to root it as it performs great. I suggest you first try upgrading to 2.3, if its not out on your phone go to
http://forum.xda-developers.com and find the forum for your phone and post there for tips, if there are custom firmwares or speed up mods available for your phone they will most like be developed by someone on that forum.