The fortress already had very bad FPS issues after cheddarius turn, due to large amounts of excess stone that had been building up over the years, Di cleaned up most of that, which made the FPS manageable again.
I think the coins are the main problem, you can't destroy owned coins like you can destroy stone, another factor may be that population levels have been rising over the last couple of years, Halltraded has been without major disasters for a couple of years, we usually had population levels below 50. But one thing to remember is to never mint coins, the economy runs better without them