Well, I think it's a bit of a stretch to put all "windows users" into a single group, but I do go on a week long Windows jag every couple of months, just to keep up with what the rest of the world is experiencing. And every time I do I'm astonished that people still put up with it.
Leaving aside the inevitable and clunky upgrade I go through; the whole system is clunky. It's not that it's slow, exactly; that would show up in benchmarks. It's just inconsistent enough you can't really get into a good working rhythm -- and this is on a relatively recent i7 processor with 16GB of RAM doing plain old office and web stuff.
But most of all the fundamental concept of Windows is hopelessly antiquated: it wants to be the switchboard for your digital life. It wants you to use it for the things nearly everyone in the civilized world is using their phone for.
To be fair, the heavier-weight Unix desktops have this problem too. Their whole concept is just wrong: the desktop is a place for getting tasks done, not juggling your life. It's not a place where you want to be interrupted or distracted, and it's especially not something you want to spend a lot of time screwing around with. Windows makes this inherent misconception worse with its relentlessly intrusive paternalism. It's constantly trying to get your attention, to redirect you to Microsoft (or partners') services and products.
Windows (and KDE and Gnome) would be much better if they simply tried to do less; if it just managed the hardware, the screen, and interprocess communication, rather than trying to manage the user. But of course, that's the whole point for Microsoft; its a vantage point from which it can sell to nearly everyone in the world who uses a computer -- or sell those people to other vendors. Google does the same thing, but the architecture of their sales effort is so much slicker it feels less intrusive (although it gets creepy when you start to notice it).
When I set up computers for other people I usually I set them up with XFCE, and not one person has ever asked for news stories or valuable offers to pop up in their start menu, or any of the other Windows 10 bells and whistles. I myself find even XFCE overkill; I use the i3 tiling window manager, which is admittedly clunky, but it a bounded, finite, very small amount of clunkiness. Learning to deal with that modest dose of clunkiness is a small price to pay for a desktop environment that starts instantly, consumes almost no resources, including my attention.