The biggest time killer in the end game is slow F1 screen. I would go to do some adjustment and it'd take Dom2 10 seconds just return back after changing tax rate for example. It works fast on first provinces, but after 150th province it gets very slow (it seems to strongly depend on how far down the list you are). Another annoying thing is small item storage which causes me to move all available items to mages for storage, so that there's room for the items being forged. But of course, it's getting hard to find the right item when it is kept by some mage somewhere
I wish Illwinter would increase inventory size. By the look of it it doesn't look like a complex task.