I have Windows 10 on what is now my "garage computer". It runs WAY better than it really has a right to. Current specs, some of which are stock and some of which I've changed over time:
AMD Athlon 4450e CPU
2 GB DDR2 400 MHz RAM
Onboard nVidia nForce 6100 GPU/chipset
WD 1 TB 7,200 RPM HDD
D-Link USB wi-fi dongle
I scored this computer for free from an "e waste" event we had at work. I used free RAM that I've scrounged from similar throw-aways at work and the WD HDD is one that I bought many years ago for 60 bucks with a coupon from Dell. I've spent exactly $0 on this computer getting it up and running.
It had Windows XP and it ran pretty sloppy with that. I'm blown away with how well it runs on 10. I've got Office 2013 on it and SQL Server 2008 R2 and run a few databases on it for work projects. I use it for YouTubing and Pandora and Alldatadiy.com out in the garage. In fact, I'm typing on it right now, with the garage doors open, listening to my neighbor mow his lawn.
It runs full-screen YouTube/Netflix/Hulu videos just great. Office '13 runs quick and snappy. Despite being a mechanical HDD, it boots from stone-cold off into the Windows environment in about 15 seconds. It runs nice and cool, with a substantial AMD cooler on the CPU and an additional 80 or 90 mm case fan that I added out of my parts box. I have some decent Gateway speakers (that provide a surprisingly good sound) that I bought from Craigslist for $5 a few years ago. Paired with a little 19" Samsung flatscreen/widescreen from another computer project I had. Great little machine for free. And great news that Microsoft will, allegedly, convert all Technical Preview installs to the real deal in July or whenever Windows 10 goes live.
10 runs at least as fast as XP on this machine. It runs far faster than 7 on one of my older laptops. It seems to take the performance benefits that 8 offers and advances them another step. Will I grow to love 10's Start Menu? I don't know. If I don't, I have no doubt that Start8 and others will develop "Start10" or similar products for Windows 10.