Backup 5x slower after adding exclude directory

Running version 0.12.1 against Backblaze (B2 API), backups used to take about 45-60 minutes with 200-400MB of changes added. Total repository size was about 1.56TB, but new repository size is about 1.06TB. I recently excluded a ~500GB directory with many thousands of files, but now backups take about 4 hours.

Is there a good way for me to determine which stage of the backup is having performance issues? Anything I can potentially do to speed this up again?

I have tried forget/prune, removing over 600 snapshots but I still have about 93 snapshots which go back about 3 years.

Discussion is continuing in the cross-post over at: