It was, eventually. In the beginning 10 was literally just Windows 8.1 (it even ran the same NT6 kernel) but with the classic UI slapped back on. They called it 10 to get away from the Windows 8 branding that everyone hated.

I recall it being pretty mediocre at release, just a reskinned 8.1. 10 started to come into its own much later after NT10

Aside from the start menu no, not really. Windows 8 is the most performant operating system. No laggy animations (thanks to DirectUI), fast boot time, especially fast on older systems. Windows 10 started the whole lagfest.

Windows 8 was ultra stable. I've seen uptime well over multiple years on it. The original UX was beyond awful and 8.1 made it ok but the core of the OS was solid.

Idk, I literally skipped Windows 8.