Xcom 2 is having some performance issues on certain computers, and players are experiencing issues with random frame-rate drops for no reason, along with lots of other issues, according to a number of posts on the Steam and the official forums.
The game hasn't had much news post-launch from the developers, but the community manager has come out in a http://forums.2k.com/showthread.php?4212311-FAQ-Basic-Tech-Troubleshooting">forum post</a> and said that the devs are "aware some players have experienced performance issues" whilst playing the game, and they are being looked into.
The community manager went on to say that they "want everyone to have the best experience possible, and will continue supporting the title with upcoming patches." In the meantime, the post gives a great deal of troubleshooting methods to get your game running a little better or for other issues.
Hopefully you're not experiencing any of these issues and you're free to enjoy what is an early candidate for GOTY 2016.