This engine-related bug has already been fixed in Unreal 4.22, which also brings some rendering optimizations.
Also, due to a bug in Unreal Engine 4.20, which Tourist Bus Sim got released with and is in use by Fernbus Sim Update 19, performance has dropped significantly. The team is currently working on the next updates for both Fernbus Simulator and Tourist Bus Simulator with the main focus on improving performance.
