In Bone Thugs-n-Harmony’s collaboration with Tupac, “Thug Love”, the guy in the first verse raps incredibly fast. I mean he makes the Micro Machines guy look like, ummmmm…someone that talks really slow. So, is he really doing that or did they “cut and paste” his voice like they did with that girl in the Fifth Element? I mean, these guys made a name for themselves by singing fast but, well, if you’ve heard this song you’ll know what I mean.
That’s Bizzy Bone. It’s real. He’s one of the fastest rappers I know of. You should also check out “Notorious Thugs” which he is in with Notorious BIG. He goes real fast in that one too. That song’s on “Life After Death” by Biggie.
Another rapper named “Twista” can, from what I’ve heard by him, go even faster. Check out the song “Adrenaline Rush.”
One more thought. The fact that “Thug Love” has a slow beat will also make it appear that hes going faster than he actually is.
But he’s still very fast.
Guys, there’s a clear winner here:
Rebel XD, of Chicago, rapped 674 syllables in 54.9 seconds at a recording studio in '92.
That pans out to an average of 12.2 syllables a second (on AVERAGE, remember - that means it’s likely he was going faster than that at some points). No overdubs or cutting or anything.
Try doing that right now and you’ll see how fast he was. 
Is there anywhere we can hear a clip of this or buy a CD of it? I’d be willing to bet that Bizzy gets above 12 syllables per second in the song mentioned in the OP but he doesn’t sustain it for nearly that long.
I’m still downloading the file, so I can’t comment on his speed, but you can find MP3’s of him on Audiogalaxy. Just search for Rebel XD, and you should find one called “Rebel XD - Record Breaker (Worlds Fastest Rap)”.