If it’s a landscape, not really. You can try resizing the textures, or simplifying the material, but ultimately it’s the iMac that’s the problem as those machines are really not cut out for running something like Unreal 4.
Also move the engine scalability setting to the lowest.
As far as performance goes you need to see what it is optimized for. If it is even optimized at all.
Common performance gains are disabling tessellation, limiting material complexity and number of layers on the same landscape tile.
also scaling the LOD distances manually to what performs best for your system… in general just messing around randomly with landscape parameters in details section until you find something that changes MS render for your system…
oh, and probably disabling the new layering system on the landscape as well…
Also, the screen might be set to a high resolution, 1080p is kind of standard, and if your display is higher then it’s taking a performance hit to run UE4 at that size.