Alpha 29.12-9 includes this fix so we can all switch back to the experimental branch now! Massive thanks to @Rubble and @Puma for testing it out and helping us! 
I’d like to point out what Fredrik said, running the game with a graphics device that only supports Open GL 4.0 (and most likely DirectX 9.0) will have noticeably worse performance when it comes to rendering large amounts of passengers as opposed to more modern cards. So machines built around and before 2010-2012 are in the danger zone.
Locking this thread now as the issue is solved and further issues should be handled in a new thread if one arises.