Is it normal for this to cause the game to lock up for 10-30 seconds every time the tool gets used? Or am I having some kind of weird interaction with other mods?

Edit: I guess this is only happening in Sandbox mode. Seems fine in regular gameplay.

Well priority is not really a thing in Factorio and you can influence it indirectly only. So what this mod does for a work around is it scans everything in the selected area. then if the area has ghosts it scans all other ghosts on the surface, so that it can move the matching ghosts outside of the area to a different surface.

So if you have a lot of unfinished blueprints and you prioritize for example a few boilers, it scans every other ghost if it is a boiler as well to move it. I'm not sure why would it be worse in Sandbox tho.

I tested it in my bot megabase with 330 hours of gametime and it didn't seem to lag.