Ever since launch, many multiplatform games just haven't looked as good (or run as smoothly) on the Xbox One as they have the PlayStation 4. Going forward that gap might be closed. At least a little.
From launch until now, developers had to leave some resources on the console's GPU free for Kinect integration, which had been mandatory. Now that Kinect isn't a compulsory inclusion, a recent code update for dev kits means studios are now able to free up those resources for games, which could (if used) make their titles run more smoothly, look a little sharper or even both.
Here's the relevant statement issued to Eurogamer by Microsoft
For consumers, simply unplugging Kinect will not impact the performance of Xbox One. The June SDK released to developers gives them access to additional GPU resources previously reserved for Kinect and system functions. Accessing the additional GPU resources is done by the developer, and how developers choose to access the extra GPU performance for their games is up to them.
So, as Microsoft say, how much the Xbox One's GPU performance is able to be improved in future games (remember, this has to be coded in the game itself, it's not like existing games will magically get faster by unplugging Kinect) is anyone's guess; while Microsoft tells Eurogamer this could lead to a 10% increase in GPU performance, actual gains will of course vary from developer to developer, and will depend on whether a game still needs to use Kinect or not.
Xbox One dev kits receive more GPU bandwidth [Eurogamer]