i5-8600k or an AMD Ryzen 1600 or AMD ryzen 2600

[downscc]

CPU speed really does matter. I think the sweet spot today for best performance for the dollar is the i7-8700K. The popular video card series based on the GTX-1080Ti is a solid choice too. Intel has released a special anniversary product the i7-8086K (in homage to their old 8086 chip from the 1980s) that I will pick for my next build when their prices cool off a little.

[downscc]

Okay, what is the recommended speed then? Do i need a quad core or will 6 core be better?

The LM recommended CPU is a quad core running 3.5 GHz: https://www.prepar3d.com/system-requirements/

My recommendation is to balance your expectations and budget. You ask if speed counts and the answer is yes, how much speed and number of cores depends on your budget. I gave you a recommendation for what I consider to be the best performance for dollar, it is a six core 4 GHz processor, so I'm not sure what you are asking.

[BRADRCFII]

I just built a 8700K and loaded Prepar3d V4 and a lot of ORBX. I used two Samsung 970 Pro SSD's.Frame rates are in the 40's flying PMDG. Finally, we have hardware that can run flight sims.