SP3 inrtroduces like 1 obscure new feature. thats it. Essentially its just a rollup of all the patches released since xp sp2 came out (basically in order to stop people who have to reinstall from having to download like 1gb of crap) so it should behave exactly like a fully patched xp sp2 system.
Is your laptop AMD though? XP SP3 doesnt like AMD machines...
Just to add my 2c, yes SP3 should install fine and not slow your machine at all. Unless you have one of those HP AMD machines as jbennet points out. It's a good idea to be cautious about newly released patches but SP3 has been out for a while and no major problems have surfaced as far as I am aware.
Yup. It should perform prettymuch the same as you are used to. its just a traditional service pack as opposed to a radical modification like SP2 was.
Got it on my dell. Held off for a while because MS needed to fix one of thier hotfixes (the one for intel HDA audio). It wouldnt install on SP3 but didnt seem to be included in it. Got an updated version of MS and it now works fine. Havent had any problems on the other 5 or 6 machines ive applied it to.