Update 2016s3 week 12 using iRacing's DX11
Without logical reason, FPS (frame rate) was dropping too low.
As a fix, I used the same settings (that I used with DX9 below) including turning on Adaptive Vsync.
Result: everything running smooth again. I make no claims what it will do for you.
It also allowed me to enable Render Dynamic Track (and Tire) Data options in-sim options without killing FPS.
(I prefer the Dynamic options turned off).
Note, the "program to customize" for DX11: iracingsim64dx11.exe located in c:/program files (x86)/iracing 
iracingsim64.exe was for DX9

For current member recommendations, check the forum links on my 3monitors page

The settings I use based on (old DX9) recommendations from iRacing's forum: