M18 R2 - Better performance on Quiet mode

So let me first say that this is an amazing PC, and I have been loving the performance for the most part.

Anyway, I recently got Ghost of Tsushima and I pulled out the stops and put it on performance mode. The stuttering was so bad! I disabled g-sync, put in a 162fps cap and fiddled with the 3D settings in the Nvidia control panel - nothing...

I tried undoing that and just adjusting the optimisation towards performance in Nvidia's app and still I got stuttering.

I was just about losing hope before deciding to put it on quiet so I didn't have to keep putting my headphones on and BAM - smooth as butter just like with GOW.

This surely can't be normal?

I've heard of undervolting and am thinking that maybe by putting it in performance mode the CPU is having some issue due to too much power.

I still have Dell premium support and am going to reach out to them, but to hone in on the real problems possibly at play, do you guys have any idea what could be going on?

& have you had similar issues with resolutions?