If you put your disc of The Witcher 3: Wild Hunt in your Xbox One X and play the game unpatched it will run at 60 frames per second.
The Witcher 3 was one of the only games to ship with an unlocked framerate. Simply what you're seeing, then, is the extra grunt of Microsoft's new console at work.
NX Gamer compared the unpatched performance of The Witcher 3 on Xbox One X against patched performance on Xbox One.
However, when The Witcher 3 launched it ran at 1600x900 resolution on Xbox One, and it wasn't until a subsequent patch the resolution increased to 1080p. On Xbox One X, then, the unpatched resolution is 1600x900, a barely noticeable price to pay for 60 frames per second.
The 60fps line isn't held always. There are dips in the dense city of Novigrad that you can see in the video, and presumably also in the foggy swamp of Crookback Bog, not seen in the video.
If you do decide to patch the game on Xbox One X, for the many updates and expansion content, you'll then be locked at a rock-solid 30 frames per second.
We know CD Projekt Red is working on an Xbox One X enhancement patch for The Witcher 3, and that it will be similar to the PS4 Pro Witcher 3 update, but we don't yet know exactly what it will improve nor when it will arrive. I asked CD Projekt Red again today and there is still no news to share.
There are many other games besides The Witcher 3 being enhanced for Xbox One X. Whether you should buy an Xbox One X is a question Wesley tackled recently, as did Digital Foundry in its Xbox One X review.