Ah, so you are doing one at a time, and still having issues?
Most important, be sure your system drive has minimum 20% of its capacity left as free space. Verify that right away!
If deleting renders and preferences doesn’t work, do a reinstall. Something is seriously wrong there. I’d think media being managed/external shouldn’t matter as it’s the same physical process taking place; FCPX tells macOS to move stuff to trash and delete it, then updates its own database. I would thing updating the database would be where issues are occurring. But hey, FCPX can be pretty buggy, so who knows?
I’ve deleted multiple Events in the past, 3 or 4 at a time, no more than maybe 30 pieces of media in each, and it was instantaneous, no issues. But if you are deleting one, and having this problem, yeah, FCPX has a bug, or something is slowing down the OS and drive, and the most common culprit would be the system drive not having enough free space, which can cause all sorts of wacko issues all over the place.