Just in case (I am not a all a PC specialist).
I had some issue with my laptop running Assetto corsa.
Like you, I had about 15 fps. and could barely do nothing to raise up at a decent level.
Until I discovered that my graphic card was not really "activated" for 3D application.
There was some kind of Throttle function which just didn't worked properly. The graphic card frequency was locked at a low Mhz
I needed to reinstall my graphic card drivers and the fps came back .
For some unknown reasons, the problem appears again regularly.
Just reinstall the same driver again and all works properly for some months before the problem occurs again.
(I don't have time to further check for a better solution. reinstall the driver takes me 5 minutes
) )
I have a dell inspiron 7720 17R with a core i7 and a Nvidia GT650M graphic card.
(I have about 65 fps alone or with about 10-15 AI cars on track (depends on the car and track of course)
One thing you can already checked is if your graphic card frequncy is on boost mode when assetto corsa is launched, and/or check if your laptop fan is working well (for example mine works at full capacity for a while when I launched assetto corsa)
I hope it may help
Of course, a "racing" PCwould be ideal