You’re using a MacBook or MacBook Pro? Generally speaking, the MacBook and MacBook Air are just not great machines for Unreal because they use relatively low-end consumer mobile GPUs that prioritize energy usage (aka battery life) over computing power. Even the lower-end MacBook Pros without a discrete GPU struggle when running UE4.
Honestly, if you’re pushing two displays with a mobile GPU, you’re not leaving a lot of GPU power for the app, and I’m guessing this is the source of the problem. It’s not a Mac OS thing, it’s a Mac hardware thing. Right now, with the exception of the iMac 5K and the Mac Pro, most computers being shipped by Apple just don’t have great GPUs in them. Epic has done a lot to make UE4 run better on Mac OS, but even on Windows, you can’t run it well without a good GPU, and Apple is shipping relatively few machines with powerful GPUs right now.
Apple’s focus lately has been much more on battery life and device weight. That might be the right call for their typical customer, but it’s not so good for gamers and game developers (not to mention, artists and designers)
The first thing you want to consider is knocking down the lighting and effects in the editor viewport. Whether that will get the performance into an acceptable range, I can’t say. If that doesn’t work, you might be better off getting a low-end Windows desktop with a good GPU, because getting a Mac that can run UE4 decently is going to set you back at least $2.5k.