In my experience, 1964 is the best emulator. Mupen is also quite fast (discovered it whilst trying to get Kirby64 to work, and it worked great!), Project64 seems to not be as fast. You can take the audio and video plug-ins from PJ64 (Jabo's) and use them in 1964/Mupen/etc, and you'll see a big performance increase over PJ64 usually (PJ64's core is unoptimized[read: SLOW] or something, is the apparent explanation). I hear older versions of PJ64 (v1.4) may be faster at certain games than later versions.
You can adjust speed further with CF(Counter Factor, increasing it makes the game render faster) in 1964. In PJ64, you can right-click a game, and choose Edit Game Settings from the pop-up menu, there's CF adjustment from 1 to 6.
You can access this option, in 1964, from the menu: Emulation | Counter Factor, or using shortcuts: Ctrl+1-8. 1=slowest('normal'), 8=too fast(probably). Usually ctrl+3 or ctrl+5 work great for me. Before you mess around with the CF too much, you may want to try different plug-ins or emus to get desired performance, because making the CF larger does seem to screw up some games, especially audio-related screw ups. It can making racing games really fun though.