Apparently, the suspected rumor is that Call of Duty Ghost's performance on the Xbox One is lackluster compared to the PS4. Normally, this wouldn't be such an issue, given we know the PS4 is technically superior, but this is Call of Duty; this is what people associate with the Xbox One. There's an embargo for Call of Duty Ghosts, but the embargo lifts earlier for every other version of the game, except for the Xbox One version. The performance issues, if they do exist, seem to be attibuted to the difficulty of working with the Xbox One's ESRAM.
The only "evidence" so far are various tweets by gaming journalists, most notably, Adam Sessler.
Links to twitter entries.
Jeremy Conrad is spilling info on his twitter account here. He's confirming much of what's been rumored and even commented on the status of other launch titles. He is a former journalist at IGN.