(emphasis mine)
That's a particularily misguided belief considering MacOS 11 is essentially iOS with a more capable GUI (the kernel is the same and there are even rumors of iphones running full-fledged macOS). Why on earth would it not get the same optimizations for tha same CPUs?
There is more-or-less a single OS kernel across Apple's OS's, ie Darwin. But there's a lot hidden in that "more or less".
There are different technologies that surface first on either the Mac or the iOS side for one reason or another (eg different driver model), the iOS model now being essentially moved over to macOS starting with Catalina.
More significantly the OS provides MECHANISM, but different devices choose different policies (eg whether or not to provide writing out swap) from the mechanisms that are available.
Of COURSE both platforms will pick up the same optimizations where relevant (cf, eg, the various optimizations made to the ObjC/Swift runtime this year)! But whether the OS' is "the same" is not a sound-bite question; it depends on exactly what you mean by "the OS" and exactly what you consider to matter for the purposes of "being the same".
As for scalability, this is an issue where comparing Darwin with Linux or Windows obscures more than it reveals. Apple appears to be trying to resurrect the underlying philosophy of Mach, easier to make performant now that they control the CPU and can add helper functionality to the cores when relevant.
So Apple is apparently trying to undo the monolithic BSD image on top of Mach, decomposing it into server processes. We've seen that with various security demons, with XPC, now with moving drivers into separate user-space processes. Some big ticket items like networking and VFS are still monolithic, but who knows what the ultimate plan for them is.
So right now Apple is kinda partway through a transition. Half the OS is still monolithic with all that implies
- higher performance, lower memory footprint
- horrible security, horrible RAS generally
- very difficult to modify the code
while half the OS has transitioned to the new world
- lower performance, higher memory footprint, yes BUT
- much easier to scale across multiple cores, and ultimately that may win as a more relevant advantage
- better security and RAS, and
- much easier to modify code (and thus to experiment with alternative, possibly better, ways of doing things)
Obviously this was the original Mach vision, never really realized. It was also the vision of a few experimental OS's like MS' Barrelfish (but like everything out of MS, some combination of fear of change and group territoriality killed Barrelfish and nothing much appears to have moved from it into production -- same story as the way large parts of iOS security are based on work done, then abandoned, by MS).
TL;DR
- yeah, Apple's OS performance is going to look lousy, especially for many-core, for a few years, as they modify a LOT of plumbing
- but there are good reasons to believe that the end result will be something spectacular. Fast on many-core scenarios (ie modern reality), much better RAS, and much easier to maintain and improve.