Mopping up is tedious when the game insists you do it. Once someone has ve clearly won the game it should congratulate them, give lots of juicy statistics and ideally a narrative to show how it happened, and then ask if you want to keep playing, as which player, and let you mop up or max the tech tree or trace your descendents for a 100 generations.
"Attrition" by T.N. Dupuy gives 20% for the winner and 40% for the loser as average battle causualties in the pre-gunpowder era. I'm pretty sure the sample did include Cannae and, remember, causualties includes wounded, who may recover in a few days, as well as killed, who won't -- except in fantasy.
I like the idea of having a more "cluttered" map. A pre-industrial world should have defensible hills and river fords, and at least sketch that the rural areas are important. An empty void between cities is just "Galciv with swords", I want it to have depth and reality as itself.
The caravans were moving and the lag was while trying to scroll, select, and move units. If there was lag between turns, I didn't notice.
Yes, it's the caravans (or at least the caravans are sufficient). I built/captured 18 towns with no roads, no lag, and good income. Then I built roads and in a few turns had 91 caravans, horrible lag, and deeply negative income.
Reality doesn't need to be plausible, fiction does.
Yes, the lag is really bad. A different problem related to caravans that also scales badly is that they have a maintenance cost. Since they spawn in large numbers (I had 118 for ~10 cities) this is financially fatal.