I'm working through the post-ace-point chapter in 501 and keep noticing that I'm surprised when the solutions classify a bearoff position as fast vs. slow. What are some of the criteria for deciding whether we have a fast vs. slow bearoff? Here are some ideas:

-distribution of spares? (if anything, I assume having our spares on lower points makes our bearoff faster?)
-even vs. odd-ended?
-number of points that have checkers on them?

Some examples (where the digits are the numbers of checkers on the ace-six points):
332200 is "fast"
222000 is "relatively slow"
222200 is "slow"
222220 is "slow"
22232 is "slow"
32222 is "fast"

What does it mean, fast vs slow, in this context?

I don't want to directly reproduce the text from the book, but basically all of these problems are about deciding whether to cube when your opponent has borne off ~5 more checkers than you but they're currently stuck in the bar. The general idea is that if you have a "fast" bearoff position you want to be more likely to cube, while if you have a "slow" bearoff position you should be more likely to hold off.

For example, I was surprised that 332200 is described as "fast" while 222200 is "slow" despite the first position having more checkers. After some thought, perhaps the answer is that "fast" means faster bearoff compared to other 10-checker positions like 222220.

That's right. A stripped position with two checkers on all points is "slow", in that you are probably taking off one checker per turn for at least a couple of turns. "Fast" means you'll bear off two checkers with a number of rolls. With an opposing checker on the bar but the game still up for grabs, fast positions are more likely to be doubles than a slow position with the same number of checkers.