Some very lazy guess work comments on here. I too am experiencing this. I have played seasons up to 2035, with the game still smooth. I always delete all saved games except the one being used.
Then on a newish save, 7 seasons in. The game hit a brick wall, it literally takes about half a hour...
I am in my second season in germany as dortmund and i am unable to play a random selection of first team players in the reserves for friendlies or matches, but they are eligible for the first team. They have been unable to all season. as a result i cannot send players there to get match fit...