XCOM 2 has apparently had a few performance issues since its launch last week, and Firaxis and 2K are trying to address that.
“We are aware some players have experienced performance issues while playing XCOM 2, and we are looking into them and gathering more information from users,” says Dave Hinkle, Firaxis Community Manager on the 2K forums.
The thread goes on to list off a number of known issues, and how you can work around them.
If you are having general performance issues, you are advised to ensure you have the latest display drivers installed, and check your hardware meets the minimum requirements. If this doesn’t work, you can try turning down the graphics options, with a particular focus on the anti-aliasing, and uninstalling any user created mods.