None of this addresses the strange postulate you make at the beginning - that you almost EXPECT an OS upgrade to be slower, and accept that ?
why is that sensible ? I certainly don't think it is so - in fact about the ONLY OS upgrade I'm interested are ones that make things QUICKER - not adding 100 more pieces of social media pieces of shite I have no interest for.
To stay with Apple - Snow Leopard was the last good OS upgrade - 64 bitting things and speeding things up considerably with GPU stuff, etc. Since then it's all being about adding more crap I don't want.
iOS is of course even worse. I made the mistake of 'upgrading' my ipad air to ios11 a few weeks ago..
it ran like a dog, changed or broke things all over the place - moving bits of UI around for no reason for example (e.g. ios - watch full screen video in browser, scrub bar at the top where I hold the screen..ios11 moved to BOTTOM of screen. these changes for no good reason get right on my tits.
So of course then I had to spend hours working out how to restore ios10 back on there, because fuck me, apple don't want you doing that.... finally got there, re installed all my apps, and made sure I have insufficient free space for it ever to download ios11 and bug me to upgrade again).
imho there has been a dramatic change in what 'upgrades' mean since the advent of 'apps' and it seems to affect OSs and 'real computers' too.
In the past application upgrades could be pretty much relied on to add features, never take any away and 9x out of 10 optimised and make things faster, using GPUs, multithread, etc - maybe not major jumps, but not things you had to ask yourself 'what is this going to fuck up', just 'is it worth the money'
Now, starting on mobile app upgrades started to mean:
- taking away key features you relied on
- changing the UI radically
- adding need for accounts and logins where none existed before
- breaking compatibility with older devices
and of course all while making it as difficult as possible to uninstall or revert the upgrade.
It only took this to happen a few times for me to switch off auto upgrades on my mobile devices now - I look at what the upgrade says it offers, and only if I really really need it do I upgrade.
Which of course assuming others are doign the same thing means we are all less safe, because app makers have destroyed any confidence users have in upgrades, so they don't get bug fixes, or security fixes either.
Now that attitude has expanded to OS upgrades (mobile and real computers), and real computer applications too - again lead by Apple - FCPX is a good example of this - a point release upgrade sometimes only being available WITH an upgraded OS, and then you find that it totally changes the UI, stops various plugins working, etc, etc.
It gets right on my tits.