Each of my track has multiple audio clips. If I unfreeze a track, make a small edit to one of the clips, then refreeze, does it have to reprocess every clip in the track, or just the edited ones?
Yup, it's a per-track feature, which means you cannot freeze or unfreeze individual clips on the same track. You can, however,
bounce individual clips. If you bounce to a separate "bounce track" (as opposed to an in-place bounce) and mute the original clip, you'll get the same functionality as freezing individual clips, but without the convenience of the one-click freeze/unfreeze.
I have to agree with Jonesey that 10 minutes does seem like an awfully long time to freeze a single track. You might try tweaking the BounceBufSizeMSec variable in aud.ini. The default is 0 but you can set it up to 350ms and trade off RAM usage for bounce speed.