Game running slow

Hi

I came back to Civ5 after like 2 years. After few dozens turns the game gets very slow. For example when one unit fineshes its turn it get ages before another one is active etc.
If I exit the game and load the save, its better for a while but soon it slows down as well.

I have :
Intel i5-2500 3.30 GHz
RAM 8GB
GForce GT545
Windows 10 64bit

Any idea what the cause could be ?
Thanks a lot