These registry files create a registry entry that persistently sets the priority of the GTA5.exe process. Run any of the .reg files based on your desired priority.
If you would rather create the keys yourself, check out the EXPERT METHOD guide on the thread. For your reference, here is the full list of CpuPriorityClass values for the dword value that you can change. The only thing you should be changing is the last digit. You are on your own though.
High Priority: 00000003 Above Normal Priority: 00000006 Below Normal Priority: 00000005
Idle Priority: 00000001 Normal Priority: 00000002 Realtime Priority (plz dont use this): 00000004