[WP81]Performance drop when navigation bar is visible on Lumia 640 LTE

Hi,
On the Lumia 640 LTE I can hide/show the navigation bar on the bottom of the screen. When I start the game, the navigation bar is hidden and the game is running 60 fps. When I swipe up from the bottom to show the navigation bar, the framerate drops to 35-40. If I hide the navigation bar again, fps is back to 60.

I have nailed down the performance drop to the method Gfx.WaitForPresent after profiling. When the navigation bar is hidden, the Gfx.WaitForPresent is steady around 8ms. When visible Gfx.WaitForPresent spikes to 20ms every other frame.

Does anybody know why this is happening or have a solution how to avoid the performance drop when the navigation bar is visible?

-Thore