Renaming the Scripts folder didn't help. However, I finally managed to solve the issue! It was caused by HPET; this post contained the solution for me:
The poster is running the same CPU as me; a 5800X3D. These are the steps I took to resolve the issue:
Open command prompt as administrator
Type this:
bcdedit /deletevalue useplatformclock
bcdedit /set disabledynamictick yes
Reboot PC
I had previously already disabled HPET via Device Manager but this was apparently not sufficient. After doing the above steps the "Simulation Time" in my DCS performance overlay was drastically improved; in some situations 10x faster! This not only allowed me to join large missions on multiplayer servers again; it also improved my framerate in many situations, even in single player (at Nellis AFB for example). DCS is now GPU bottlenecked again instead of CPU bottlenecked; as it should be.
Thanks for the suggestions and hopefully this post will help someone else in the future.
