Track is heavier than others, with me it's the same, while i have 120 FPS average on most tracks, in Sebrig it drops around 30 or 40 FPS... If you plan to use only this track for a while i recommend changing the PP Effects to Low or None...
I used to have this problem, although my stuttering was low FPS of around 14 - 25 with approx 10x rF2 GT3 Pack cars on the rF2 laser scanned Sebring...this was with my old GTX 690 settings below... I have since upgraded my GPU after 6 years to a RTX 2070 with no other PC upgrades and this alone made a massive difference with an increase of FPS 139 -200 with the above low settings...so I increased all the rF2 settings to 'Full' running the 2070 and still get 54-75 FPS with Rain and 10x GT3 cars at laser scanned Sebring. ...however this is my personal experience, by reducing the settings in rF2 was the only way to get smoother FPS on laser scanned Sebring.
PP Effects have an important role when it comes to FPS, more than tweaking in the graphics settings...
Remember to use the "12 hour" version and not the "full" version if your PC is not the latest. The "full" version has much more objects and will be tougher to run. I would even put shadows on "normal" if your CPU/GPU isnt very strong, and then follow @M D Gourley tips above, and if that runs good turn up Anisotropic to X16 and test. If you get stutter bring it to x8. You want a clear tarmac in front of you.
