As i'm sure you can appreciate, the game isn't complete yet; there may be some memory leaks in there, and it certainly hasn't been optimised yet. But the game also has to track a LOT more information than BOTF ever did, so there will inevitably be a slow down. The game will also likely have to track even more information by the time the game is complete, so if anything it might get worse.
For that reason, we recommend people ONLY play on tiny and small maps. Just in case it is a potential memory leak though, i'd recommend you save and reload the game if you plan on playing for more than a few hundred turns. That might clear some of the memory load and speed the game up.
I remembered the save, quit, reload trick from Botf and it works for a bit and then slows down again. I'm sure it'll get fixed in time, just wanted someone to know it was happening.