I have great news! I’ve been messing around with “Engine Scalability Settings” in both PC and mac, and there seems to be a problem with the settings “Global Illumination” and “Effects” (and especially this second option). Let me explain:
On my PC, I can put all settings on “Cinematic” and it works fine (no stutters) until I put “Effects” on any setting higher than “Medium”. As soon as I do that, I start getting stuttering again. But, even if I put “Effects” back on “Medium”, the stutter persists until I put “Global Illumination” also on medium. Very strange.
So right now, on my PC I’m getting 120 FPS (no stutters) with these settings:
And on the iMac I’m using a very similar configuration.
I’m so glad I found the issue 
