Did you try out the new CSP extension for DXGI flip model etc? Enabling that causes pretty much the same issue on my machine. Without spending some time digging for the exact issue and fix, I have to disable the extension, then restart computer to get my performance back. I am leaving this off for now, I think it didn't play nicely with my machine when it was inside the other menu either.
Also this issue has been caused for me in the past by either Sol, or WeatherFX in general, even if I cleanly reinstalled Sol or deactivated entirely CSP. Same also where I have restarted the computer as last ditch effort and the framerate comes back for a race or two then disappears again.
-Try going to WeatherFX settings in CM and switching weather script to one you are not using eg. from 'Sol' to 'Sol 1.5.x' or 'Default Implementation'. Load a session, quit and switch it back to 'Sol' (or whatever one you wish it to be on). This fixes it for me.
If not, I wish you luck in finding a solution because things like this just make me stop playing the game until I have the time for troubleshooting and that sucks because life is very busy at the moment.
Yup tried all that and didnt really fix the issue for me. Only thing that fixed it so far is after running optimization on my SSD. Knock on wood so far it's been working flawless. CSP new release is buggy, it's wonderful with all the new features and when it's working right but when it doesn't it is a real headache to troubleshoot. I remember one of the patch a while back completely broke AI. I remember in Bathrust track some of the opponents would just run straight into the wall first corner.