I think "fast" means that it can keep up with many different things that are going on in the music.
If it tends to blur some of the faster and more complex passages together, it is probably on the "slow" side.
I actually used to have a 600 a while ago.
To me, it was only marginally better than the 555's.
If I paid the $400 retail, I would have been so pissed, but for the $150 used I got it for, I didn't complain.
But the lcd-2's i just got are worth every dollar. this upgrade is the best money spent i could have possibly hoped for.