I think the problem is as solved as it can be for now. I’ll share the steps I took in case anyone else is experiencing the same thing:
1) I emptied the Mac equivalent of the Adobe temp folder. You can find it here: “Go” menu (in Finder) > Go to Folder… > /tmp. Deleting the contents of this did help somewhat, though it didn’t speed up my system as much as I would have liked. (It got rid of the spinning wheel, but there was still a delay.)
2) My raw sound is in .wav format. I converted the audio for the sequence I was working on when the problem started to .aif format, and I noticed an immediate and significant improvement. I’m not sure how this explains why the problem began when it did, but it worked!