if you can not find out, with all the informations you have, while sitting directly in front of your PC, while you checked “many things”, etc… how should we imageing with the given informations what the problem of your new system is ?
Recommendation: let run some performance test tools and compare the results and ensure in that way, that there is not a generall problem with your new pc ( hardware, e.g temps, or a setting ).
Check the mainboard drivers and all that stuff, I know as own experiance that some z790 are electro-waste , nothing else. Also have an eye on the 990 firmware, there was a horrible bug. And I cant never understand why users disable HT. Otherwise million possible reasons…