I've just edited the config file and it boosted my FPS from 30 to 50+
In file are two version of the config. Reflection on and reflection off. I recmmend to use version with ref. on, because ref. off version has a little bit glitchy light on cars.
In the settings you can change whatever you want to, but if it says custom and you'll change it, you won't be able to change it back to custom, only by replacing the config file again. You would want change your resolution, because I'm using 1024x768 (FPS for the win!)
Just copy and replace one of the file into "\Documents\My Games\F1 2015\hardwaresettings"
You don't have to backup the original one... If you run the game without that config, it creates one.
Please, let me know if it helped to you.
Sorry for my english.