So far my installation of Windows 10 performed smoothly.
Booting time is fantastic even just with a small SSD used as cache drive with Intel SRT (everyone should use this if you are not running SSD-only, have a spare SATA 6Gb/s port and your hardware support this, and the 64GB cache drive capacity cap means you can use those cheap Kingston 60GB SSD for this.)
For smooth video playback you will need some video card. If you have a recent Intel processor the integrated Intel HD Graphics, if you have it, should be able to hold up. My machine used a Xeon E3-1231v3 (BTW, when you are buying your next desktop computer, keep an eye on those Xeon E3-1230-Series, they can have really good cost-performance ratio if you use dedicated graphics) that does not have integrated graphics, but the GTX 650 Ti graphics card held up relatively well.
One downside so far, maybe a weird bug, start button can stop responding (just the button, not the entire system) after a few days of continuous use.