Calender is too slow in Manager Career

I get between 100-120 FPS. But there seems to be a delay in the menus. Especially on the calendar, time passes very, very slowly. While 1 day passes in 2 seconds, now I wait 30-40 seconds. It's just starting to happen.
Since I only play offline, I don't know about the online part. I turn on my internet and enter Steam. I open the game and turn off my internet. But the simulation is no longer playable because it takes too long.

I just noticed. Although the game is fast at first, it slows down later. When I exit the game and verify the files on Steam, it downloads 3 files at a time. I've tried this a few times and it's the same every time. I don't know what these files are, but how can I solve this problem?