Thanks for your comments.

maybe you have chosen a wrong backbuffer format?
No, I've tried that too. I tried it with default format (presumably the same as desktop format). and then I've tried to set backbuffer format to a number of different values. No effect whatsoever.

VSync ?
Swapeffect is set to discard. But even if VSync is on, refresh rate of my monitor is 60 and the application is very simple. But I get around 30FPS.