Not if the thing is CPU limited when that many cars are involved (physics simulation for every car can be costly)
That's what I was thinking. Racing phsyics-engines run on higher framerates (Forza 5's engine is 300+ fps if I remember right) and PCars has so many cars rendered... many devs have called the CPUs in this gens consoles a bottleneck, so I directly jumped to the same conclusion, that it's a CPU-limitation problem.
But thinking about it, if the framerate holds up in clear weather conditions, then it's probably not a CPU-limitation.
The best solution for me would be if the game would guess ahead if it will run into low-fps trouble in the race you selected and then give you options like...
"constant 30fps",
"less (weather) effects",
"lower internal resolution",
"go back to race options",
"run it anyway",
"Idk, let PCars decide for me".
But who knows, maybe it really was that this build wasn't ready for technical analysis and the final version will run at smooth 60fps without much of a downgrade.