Thanks for reporting this issue. Before we proceed, I have a few questions.
Are you able observe the same issue in release builds?
Does this issue reproduce in LunarG's cube.exe? (uses Immediate Presents)
Does the slowdown scale? If you gave it 3 milliseconds of work would the second submission take 6 milliseconds?
Did you have any other graphics applications running in the background while observing this? On my Carrizo setup, I saw the 2nd submission decrease in time, probably due to warmer caches.
Any further info you can offer would be appreciated.
Derrick