Guild Wars 2

This one is pretty straightforward to reproduce.

• Have a guild with 400+ members or a very large contact list.
• Open the guild roster or the contact UI.
• FPS of the whole game drops to sub 10 FPS.

You can even see players or NPCs in the background “stutter” while the two mentioned windows are open, and the UI becomes very “laggy” and unresponsive. Game returns to normal after you close either of the windows in question.

This happens to me too with a large block list. It makes it very troublesome to check if friends are online.

My FPS drops to ~5-15 when the window is open if there is any player/camera movement.

Seems like they need to optimize information gathering with large number of players for the roster/contact lists.