Performance problem with plataforms like game

i’ve been asked to make a site like , i already know how to do everything but my problem is the preformance of movement,
i did a game that has a similar navigation like ff0000 but testing it with a Pentium3 looks a little slow while ff0000 in the same machine does not.

my question is where to find information about this type of code for AS2 (tutorials, snippets etc…) ??

I have tryied searching for a good physics engine for AS2 but i just founded for AS3.

Any suggestions ?