Some games hit 4K with extra GPU overhead, others struggle with higher resolution.

https://youtu.be/R_7iD9wjdCE

When we got our first hands-on (and eyes-on) experience with the Xbox One X at E3 last month, we weren't exactly in awe of the improved resolutions and graphical effects on display. Now, though, we're starting to see initial reports of just how "the most powerful console ever" improves raw performance on games originally designed for the base Xbox One system.
In an extensive report, Digital Foundry cites development sources briefed by Microsoft earlier in the year as the site outlines performance information for nine titles running on Xbox One and as simple ports to take advantage of the Xbox One X. While Digital Foundry wasn't able to confirm which specific titles were being discussed, data on the genre, engine, and resolution targets allows for some educated guesses regarding the nature of these titles.

The site stresses that these are rudimentary, unoptimized ports that don't make use of any new hardware effects on the Xbox One X, nor do these ports account for changes like the move from a fast ESRAM cache to slower GDDR5. As such, the numbers under discussion here "may be underestimating the real life result on Xbox One X," as Digital Foundry's Richard Leadbetter puts it.

How well these basic ports are able to run at full 4K resolution on the Xbox One X seems to depend largely on the resolution they can already achieve on the base hardware. Two games that run at 1080p on the Xbox One, for instance, actually use less GPU time to run the same content at the same frame rate and at a native 4K resolution on the Xbox One X. That extra GPU overhead can be used for visual upgrades that go beyond extra pixels, like more impressive lighting effects or detailed models

Games that run at 900p or 720p on the Xbox One, on the other hand, seem to have more trouble reaching the "full 4K" standard Microsoft is hyping for the Xbox One X. One title went from a 720p, 60 fps performance on the base hardware to a 4K, 38 fps performance when trying to force peak resolution on a basic Xbox One X port. For games like this, techniques like checkerboarding and dynamic scaling might be needed to maintain framerates at higher resolutions even with the Xbox One X's additional power.
Digital Foundry's report and accompanying video go into a lot more detail on the reported Xbox One X performance, including improvements to Xbox One games that aren't specifically ported to take advantage of the Xbox One X. Microsoft's apparent advice to Xbox One X developers, as relayed by Digital Foundry, provides a good summary of the performance boost: "Expect an easy port to Xbox One X with a baseline 4x resolution boost, start with your high-end PC settings, use the memory well and fill any extra frame-time with additional effects."