PC very stuttery/laggy on horizon 3

You can track your utilization with software like MSI Afterburner (no matter what brand your GPU is !) to check for CPU, RAM, GPU, etc.)

Someone asked for FM7 today … with a GTX 1060 3gb gpu too.
https://forums.forza.net/turn10_postst114770_PC--Low-framerate-and-weird-performance-behavior.aspx

Just did a quick check for FH3 …

1080p 60fps @ everything on lowest: min 2874, max 3100+
1080p 60fps @ everything on ultra: min 4500, max 5000+

Same GPU, different game, same problem, imho …

Test it for your system and post a screenshot.

I recommend to log CPU for every core, GPU usage, GPU power, GPU memory usage, and RAM usage.
See my screenshot inside the other thread.

It will log the utilization in the background while you are playing.