I found this interesting nugget to enable win10 ultimate-performance-mode on my laptop...

https://www.windowslatest.com/2018/04/20/enable-ultimate-performance-mode-in-windows-10

it makes my laptop fan stay on...
It did not help the code execution time while I had a browser open
I will try with the browser closed.

Not too much of a difference with browser closed
What really helped to stabilize my results was Turning OFF WIFI

Even then...
There is still too much variance to determine if a new looping structure using a large pre-processed list
is better than 2 loops that created the same list on the fly (using foreach and keys)

I will look more into Benchmark as suggested by bliako