Moving this to bugs so we look into it further. It appears to have stopped during a permission rebuild (which can be an intensive process), though it may have spent 95% of the time stuck elsewhere.
I dumped my test site and started over from scratch.
This time i did import the posts and threads, and i also used Paul M's import tool instead.
The import kept dying at random spots, had to refresh it a lot of times, took a few hours.
But that was nothing.
Since the problems continued after the import and the automatic indexing that happens after the import, i decided to run the index tool on everything one by one.
Now THAT was a chore. Specially when it got to threads/posts.
I spent most of the evening doing that, all through the night and into the morning.
It kept stalling/timing out at random times, sometimes every 15-20 minutes or so, then it would be good for an hour, then another rash of it crashing.
Eventually it was finished.
And now stuff does delete, yay!
But it takes a looong time to do so, and it never leaves the "deleting" overlay in the ACP.
It just stays working on the deletion forever. But after a good chunk of time has passed and you hit cancel then refresh the page... the section you wanted gone is now deleted.
I know my DB is kinda big, but deleting stuff works fine in vBulletin, there's no reason it should be this slow with XF.
Even creating/importing styles and deleting brand new forums or pages takes a good 3 to 5 minutes. That should be quick.
Before i do an import everything seems to work fine.