Something definitely weird going on…your system specs even with gtx1080 shouldn’t be having issues.
I have personally installed and tested/played on a system with a old i5 (2010 or older) with 32gb ram and a gtx1080 almost all settings on high @2160x1440 and it is overall smooth gameplay and no lag going through and to the menu’s.
Do you have the specs for you ram (size, manufacturer CL speed) the 16gb vs the 8gb chips…are they same speed?
Have you monitored your temps while simming?
And sorry if already asked…did you set game exe to run at high priority?
Finally how many programs do you have set to run at startup of windows?