Really not sure on your specs but looking it up what you put and the issues it sounds like you're pegging out on ram and vram. I don't know if you can upgrade anything or not (I know Macs typically aren't... but maybe they are lol I'm really not familiar with Macs), but extra ram and, certainly if you can, upgrade the GPU in it (if it has a dedicated card in there, I think that's a chip onboard). From a google on 2013 iMac, it's got a GTXM that has 2gb vram, and your 8gb of system ram is running 1600 mhz, so those are two bottlenecks.
If not... lower your texture size to half or quarter, utilize dynamic resolution (set it to scale, not auto), and perhaps lower the terrain quality a bit.