Game can be windowed, fullscreen or fake fullscreen. Check video options. You also don't have to select low quality - your video is capable of high+ graphics in 1440 definitely.
Your other apps generally don't have an effect on divinity unless they actively mess up video card:
Google chrome
Adobe flash
Keynote
Those should better be closed.
Try fake fs please and report. People report easy switching from low to ultra on 6xx cards sometimes.
