I have a Mac mini end 2014, 2,6GHz Intel Core i5, 8GB 1600 MHz DDR3 with Mojave.
I spend hours (well ok, minutes) watching the coloured spinning wheel whenever I'm launching something, or even coming back to a window. Memory is never fully used, nor the CPU.
It's specially slow when I start a session.
Is my Mac just too slow for Mojave?