It takes sometimes 3-4 minutes, sometimes up to 20min or more for it to eventually stop. If I issue the stop once and wait, it will eventually stop. But if I try to kill the container or the process, it can get stuck for much longer.
I don't notice anything strange with cpu utilization. During preclear, core 0 seems to be pegged at 100% the whole time and the others are pretty much idle (I have 16 virtual cores).
I'm only preclearing 1 drive at a time and have plenty of ram (64GB), with only 10% utilization.
Actually, now that I tried it a bunch more times, I noticed that with just the preclear, on unraid dashboard, the core 0 utilization bounces between 80% and 100%, but if I try to stop a container, it will remain at 100% constantly. Perhaps it is a cpu bottleneck where both processes hammer core 0 at the same time.