I struggled with this for a long time (having an iMac as well) and have only found two things that helped with performance: An SSD and controlling temperature. I have a mid-2011 iMac and I can run 60fps low-medium settings (mostly) except in meta events. After I put an SSD in my iMac, load times were down to a few seconds and having my fans control temperature better reduced frame drop. I'm sure these would hold true for a newer iMac.