At launch, it came with Kinect and 10% of system resources were reserved for Kinect processing, even on games that didn't support Kinect. That resulted in lower framerates and resolution than equivalent PS4 games.

Then Microsoft, wisely, removed the Kinect requirement and released a Kinect-free version of the one. With that extra performance boost, the One gained parity with the PS4.

Sony announced the PS4 Pro for 2016, but while it had more power than the stock PS4, it lacked a 4K Blu Ray drive.

Seeing the opportunity, Microsoft added a 4K drive to the Xbox One and launched the Xbox One S one month ahead of the PS4 Pro.

They also pre-emptively announced the Xbox One X which would be the powerhouse machine of the generation with 4K gaming and 4K physical media.

The idea being that hopefully people would choose the One S over the Pro due to the 4K drive, or would at least wait on buying anything until the One X dropped a year later.

Last generation was really weird as to one company having both the weakest and strongest hardware in the same generation.

Xbox One W/ Kinect
PS4 / Xbox One No Kinect
Xbox One S (same hardware + 4K Blu Ray)
PS4 Pro (stronger hardware, no 4K Blu Ray)
Xbox One X (strongest hardware + 4K Blu Ray)