This is one of the huge challenges with this kind of testing. There will always be a scenario in a game that will be more demanding than others, but in this case, the performance loss is pretty extreme.
Favorable weather conditions can cause minimum framerates to increase by as much as 45%.
In all the game is extremely scalable and is very easy to run in most situations, though it can really make your PC cry if using unfavourable racing conditions at the game's highest settings.
I kinda wish that I had Bernard's watch or something similar so that this kind of testing could be done within a more reasonable timeframe.