The thing is that I have different OS installed on external SSD. El Capitan, Sierra, High Sierra, Mojave and Catalina. Mountain Lion still installed on the internal Fusion Drive of the iMac that came with the iMac in 2013. From my own experience, all OS work very fine but Mojave tops them all. As a footnote, whenever I feel nostalgic, I boot the iMac with Mountain Lion. First, it's blazing fast. Nothing slows or hangs. All native apps works like a charm. All but one and guess which one it is: of course Safari. This app is a real pain in the neck if not in the derrière.