In my case FPS din't drop so down) just from 90 to 40.
What save me in my case:
Try to
Create a new windows profile.(new windows user)
Unistall AC & STEAM. Install in new location where no one of mods was added.
BE discipline - not install everyone mod you see find. Install only that you need now!
For this i use Content Manager. Content -> mods.
Put your mods in this way:
%your assetto corsa folder%\mods\%name of mod as you like%\content\cars\%copy of mod floder%
Some times you need to create \content\cars\ or \content\tracks\ folders sometimes it already in archire that you download. And enable what you need just for now in a same tab (Content -> mods)
Think about -maybe you dont realy need SOL mod. Instead you can enable magick.NET in settings -> content manager -> plugins.
Oh! And disable ALL applications that record video in background! for example "GAMES"
The game mod does not help at all so turn it off too.
AND) Try to start x86 version of game! In my case it help a lot with Wreckfest.
If it does not helps.Totaly unistall & install new video drivers and reset settings to factory defaults.
Good luck!