So, the Comet Lake CPU in the 2020 iMacs is the process node from 2014 (Broadwell, 14nm) with the microarchitecture from 2015 (Skylake), with some minor improvements.
It’s a bit like Apple taking the A9 and making an A9Z variant which adds more cores, in 2020.
Now, Intel is working on fixing this: Ice Lake and Tiger Lake are 10nm, and each add major microarchitecture changes. Rocket Lake, some time this winter, will then take some of Tiger Lake and retrofit it in another 14nm CPU. So that’s when, hopefully, Intel will gain some major performance on their desktop (and high-end laptop) CPUs again. But not today.
To be clear, I also think praising how many cores the first ARM Macs are rumored to have is a little misguided and premature. Perhaps Apple will do something similar to Turbo Boost, but if not, I would expect an 8+4-Core arrangement on a low-end MacBook to have a lot of idling cores. The code that takes heavy advantage of parallelism simply isn’t there. In specialized applications like compiling and video editing, sure, but those aren’t typical uses of a 12-inch MacBook.
My guess is either these rumors are wrong, or there’s another shoe to drop, like Turbo Boost.