Unless one go quickly for the victory (like conquest) on tiny map, the late game is very bad. There are 2 problems
1) Technical problem - the game just become VERY slow. I have good computer, I do not think it should slow down like that. More over, it seems that it just hits some kind of barrier, like in 20 turns around turn 150 it converts from OK to run game, to nearly impossible to run.
2) Late game mechanics. When you kingdom is big, it becomes just way too tiresome to play. I am talking about constant attacks of caravans that require clean up forces EVERYWHERE. And if you have a war and some quests on your hands, the whole game becomes just too much. It is TOO much going on, too many UNINTERESTING things to do. (train unit, train pioneer, do not forget about those, send them together, oh no - to many monsters, find another unit go clean the aria, found the sity, build up the city, reapeat, while at the same time, protect the caravans from monsters from every directions, train inits, build buildings in your 10 or so cites, it becomes really uninteresting, cumbersome with no interesting decisions to make.)
I think one big help could have been, if monsters would not be able to enter your territory at all! Like in civ games. Also, their amount must be reduced. Also, teleport spell must be cheaper, and/or, there should be away to prevent spent mana in auto combat so that it goes below some amount that you can specify. There may be another teleport spell require research, so it is like level 4 spell, that let you teleport to any place where you do not have FOV. So you could teleport and protect pioneer. More over, heroes should be able to teleport as well, not just your main.
There must be also some kind of mechanism to prevent building so many cites. Even when you conquer new ones, you should be able to destroy city because it is too hard to maintain or something. Because maintaining 10+ cities is ugly. Or there may be another way... like making possible to build just ONE city building of ANY kind (may be excluding housing) per WHOLE KINGDOM. No monastery in every city! Or there may be some mechanism, like doubling the price of building if it is build in second city, tripling for third, and so on...