Freezing Multiple Audio Tracks

You freeze synths, not audio tracks.  This releases the RAM that the synthesizer uses to process the MIDI data.
What are you trying to accomplish with freezing the audio?

How do you freeze multiple audio tracks simultaneously? I've tried highlighting several tracks and choosing Tracks - Freeze, but that doesn't do anything. I end up having to click on an individual track, click the freeze icon, wait 10 minutes for it to freeze, and then go onto the next track. It is very tedious. I'm using 8.5.

That, unfortunately, is how it must be done. We've been asking for a group freeze (you should be able to select multiple tracks and freeze them all at once, or perhaps a freeze button on track folder headers) for almost as long as there has been a freeze function. Every year, I hold out hope that maybe this is the year we get it, only to be disappointed again. But I'm still optimistic about X2!

I end up having to click on an individual track, click the freeze icon, wait 10 minutes for it to freeze, and then go onto the next track

This doesn't sound right to me.

Most of my synth tracks will freeze in under 30 seconds.

The only ones that don't are synths like BFD2 which can take an absolute age to freeze properly.

Bounce the A/S clips down would be my suggestion.

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.

Please add group freeze and thaw feature!!!