Not too many really fast mechs out there. I do remember people doind some crazy things to some mecha and actually being able to walk them, but that is quite few and far between.
Fast is determined by quite a few factors. Newer mechanical spyders are pretty fast, 5-8 bps i'd estimate, so are some cockers and tippmanns too. But you have to have the hopper. Gravity killed rof a long time ago.
fastest is definitely the automag.You can essentially walk the trigger like you can on an electronic frame once you set it up right. It's also the best quality as far as most anyone is concerned. autocockers are a close second, depending on what you get though.