While windowed game is in the background (not the target of focus or capture), the game continues to track /capture mouse movement and provide tooltip information in-game for your current cursor coordinates.
As an example, while paused in windowed full-screen and typing this message into the browser window, I can mouse down over "drag files here to attach" on your form and see the lower half of a tooltip pop up on the periphery. Moving my cursor around in this form's window as I type provides me with real-time change between tooltips in 25% of my screen that remains visible behind the browser. There is not any delay or performance decrease while accessing this tooltip information, as though I were still in-game.
I suspect this accounts at least partially for the high dynamic resource usage I've been seeing even when the game is paused and minimized.
Steps to Reproduce
Windowed full-screen the game. Then tab out and begin using another program over top of it, such that you can still see some of the game in the background. Nudge the mouse around without clicking and you should see portions of the larger tooltips pop up around the edge of your focused program.
This issue has not been confirmed by a developer yet.