They are either iPad Air 2 or iPhone 6 and up to a certain extent iPhone 5S users.
iPhone 4S, 5, 5c, 6 Plus, iPad 2, 3, Mini, Air 1 will not ever say it is smooth.
I pity Air 1 and Mini 2,3 owners. Apple had paraded that A7 was so fast SOC but a little bit of transparency/blur rendered it obsolete.
I concur that for us to have lag free/stutter free experience, Apple must completely remove this transparency/blur effect from iOS and come up with better design which is less resource intensive. But instead, they keep on adding extra layers of blur at every conceivable place and we get those random stutters. Blur is also battery sucking as it makes GPU to work little harder.
That is not a lag caused by Windows itself. It's the third party app or some flop antivirus like McAfee which bogs down those Surface tablets.
Microsoft knows that this installing of apps which mess with registry and system files causes all kinds of problems to users. But at the same time, the apps from their app store pale in comparison to fully featured desktop style apps.
I installed Windows 10 on MBP 2009 which has Nvidia GeForce 9400M. Damn it runs smooth with perfect transition animations and blur even.
But install El Capitan on it and the whole system becomes lagwagon