Auto-Stack By Capture Time is extremely slow in my LR 9.3 but is nearly instantaneous in LR 8.2.1.
In LR 9.3 / Mac OS 10.15.5, I selected 1185 photos and did Photo > Stacking > Auto-Stack By Capture Time. I got the spinning beach ball for over two minutes and LR was unresponsive before the dialog finally showed how many photos would be stacked:
In LR 8.2.1, I selected mostly the same photos (1170), and Auto-Stack instantaneously showed me how many photos would be stacked, with no spinning beach ball.
Something broke since LR 8.2.1. Auto-Stack should be just a simple sort and linear scan of the capture times of the selected photos, so the behavior in LR 9.3 is clearly a bug.
Updates for the Adobe Photography Products were released on 10.20.2020 that include fixes for these issues. Please install the most recent update and confirm that your issue is now fixed. Please let us know if you encounter any issues.