Viewport performance in Scene view when using Linear colorspace with Forward rendering is much lower than Gamma.

When using Gamma, rotating viewport is smooth, in Linear viewport has obvious stutters and lag with even simple scenes
(tested on Windows 10, both on AMD and NVIDIA)

Increasing Scene view window size worsens the performance considerably

Moving Scene view, resizing or detaching tab can take over 5 sec

Repro:

1. Open the attached project or make a new project with a default terrain
2. Set color space to Linear mode (Project Settings->Player)
3. Detach Scene view and maximize the viewport
4. Open the Profiler window and enable Profile Editor
5. Hold R-mouse to rotate view while moving around with the arrow keys
6. Notice as you fly around (especially when whole terrain is in view) there is a lot of stuttering and lagging in the viewport
7. Notice the profiler shows a lot of green spikes causes by Gfx.WaitForPresent using 92% frame time
8. Switch to Gamma Color space
9. Notice no slowdown occurs when moving around in viewport and profiler shows no spikes

NB: disabling 'AllowHDR' on scene camera makes the issue disappear.

Works as expected in:
5.6.0a1, 5.5.0p1

Problem happens in:
5.6.0a3+

Regression has happened somewhere between 5.6 a2 and a3