1. Which version of OMSI 1 or OMSI 2 is being used?
2. How often does the problem occur and how? (Error message, but playable / Crash / Freeze / ...)
All the time. Whenever I load up a game, it usually starts at around 50-60FPS around most of the map, and even if I start in a very resource-heavy area it dips down to only 40FPS, but as I drive along, the FPS keeps falling gradually until it becomes 15FPS or lower and is unplayable. Restarting the game with the exact same scenario and save will "restore" the FPS, but it will again slowly drop down.
3. Which action leads to the problem? (Place a new bus / Driving on schedule / ...)
Driving on schedule
4. Which content was used? (Map, busses, ...)
All maps and buses, but most recently Berlin X10 and VanHool AGG300 (1998)
5. Which mods or plugins were used? (SweetFX, ...)
6. Logfile in spoiler or attached
7. System information (OS, RAM, CPU, ...)
OS: Windows 10 20H2
CPU: Ryzen 9 3900X
RAM: 64GB DDR4-3466 CL14
GPU: RTX 2080 Ti FE
Storage: Corsair MP510 M.2 NVMe