So, at what point are all those ajdustments to RAW files in ACR 'saved' or lost?
My understanding is that, if using sidecar files, then changes are written to them asap. If there's a 'bulk' change done then you may see a progress indicator. Assuming you use sidecar files have a look at some file timestamps. What I would do is kill the cache for this directory and see what happens. It is the rebuilding or 'applying' the changes that takes time.