Are you experiencing or just a consistently low frame rate ?

The in-game graphics menu only allows for a limited range of adjustments. By editing the user.cfg file, you can access hidden settings, disable advanced effects (like reflections, dynamic shadows, and high-quality motion blur) that are not fully turned off by the "Low" setting.

This turns NFS Payback from "unplayable" into a stable 30 FPS experience on hardware that is 8 years old. Don't expect 60 FPS, but it beats watching a slideshow.

By modifying this file directly, you can bypass the game's built-in limits. You can disable hidden, resource-heavy graphics features like advanced ambient occlusion, high-quality motion blur, and heavy shadow cascades that are otherwise impossible to turn off using the standard in-game menus. Where to Find the Configuration File

The in-game slider only goes down to 50% (720p on a 1080p screen). You can go lower manually.

If your FPS fluctuates wildly between 20 and 40, capping it at 30 using software like RivaTuner will provide a much smoother experience.