Hello. I have a late 2012 iMac 27 - 24 RAM - 1TB FD - 3,2 GHz Intel Core i5 - NVIDIA GeForce GTX 675MX 1024 Mo. I have these OS installed on external SSD : El Capitan, Sierra, High Sierra, Mojave. Mountain Lion is still installed on the internal 1TB Fusion Drive. Fastest OS for my old iMac is in this order: Mountain Lion, El Capitan, Sierra, High Sierra, Mojave. So, the slowest is Mojave. But it works, don't worry. Hard to see the difference between High Sierra and Mojave but easy to between Mojave and Sierra and El Capitan. Most difficult tasks for my iMac are of graphical nature like drag & drop pieces of texts or pictures on the desktop. Photos is definitely slower but old apps like Aperture and Lightroom are blazing fast surprisingly. Some apps don't open like some add-ons for Lightroom like Viveza and the likes. As for the dark mode, I can live without. It's very obtrusive being all over the place. Still prefer the dark Menubar and Dock of the other OS. Being wise, I'd stick to El Capitan or Sierra but Im not and I'm on Mojave more of the time. But I also find myself switching back to Sierra or High Sierra some times. Too bad, Mountain Lion is half usable now. It was blazing fast for this 2012 iMac. Now you've got the whole picture my friend. R.