i think the fluidity more than the speed is what's key, because it's pretty much spike and the gang's philosophy. easy come, easy go. everything flows with bebop, the characters take life as it comes.
keanu, on the other hand, is one of the guys who always seems uncomfortable in his skin. he possesses a sort of uneven aura - pausing, stuttering, which in a way is why he was perfect for the matrix. he was a man out of his element. the antithesis of everything spike stands for.
so like i said, the irony is so delicious this has to happen.