I have an ATI 1950xtx and I play windowed so I can quickly get to a browser. ATI's 3d drivers have a weird 2-3s pause that gets caused when new window regions are created (like tooltips). The curse client "notifications" and "download" windows cause a pause in the game UI each time they popup. I don't have trouble with other clients, because they stay in their own windows and don't do anything while the game is running.

Any chance to add an option to "disable all ui popups while game is running" or something like that?

Yeah I definitely need to find a way to solve it for all of the windowed mode players.Right now it only avoids displaying the notifications if the window size is the exact same as the resolution of the screen.

Tbh I never considered it an issue that people would get notified while in windowed mode but this issue is brought up a lot so it's obvious that it's not something which is desirable.