To make the game use all CPU cores, we need to add "+fullproc" to the game's parameters. To do that, open the rFactor properties tab in steam and click on "Set Launch Options". Then enter "+fullproc".


This alone can double your fps, so the next step isn't really neccessary, but it can't hurt, can it?

I found this second step unneccessary, but I have heard of people benefitting from doing it, so I'll include it.

To give rFactor high priority access to system resources, do the following:
1. Have rFactor open.
2. Open the Windows taskmanager with Ctrl+Alt+Del.
3. Go to "Details" tab.
4. Rightclick "rFactor.exe", select "Set Priority" --> "High".

Now go drive some cars and enjoy the good performance!