[PBR] Sudden and large loss of FPS as sun sets

Repro:

1. Get the day cycle from the object at http://maps.secondlife.com/secondlife/Danger!%20Danger!/72/83/39 (The relevant keyframes are the last 2 keyframes, normally positioned at 88% & 94% in a New Day Cycle, but shifted in this example day cycle)
2. Look out to a sunset/moonset over the sea.. e.g. on the public bench on the beach at http://maps.secondlife.com/secondlife/Damiano/49/6/21
3. Open the Day Cycle
4. Watch the framerate as you toggle between the last 2 keyframes in the Day Cycle.

Observed: The framerate fluctuates on some machines.

Expected: The framerate should be constant.

(Note: This bug does not repro on my machines so I hope it reproduces on a Developer's machine. -Dan)

Bug copied from https://jira.secondlife.com/browse/BUG-234859

What just happened?

Using Second Life on Bellisseria as sun set. I had the Statistics bar up for PBR viewer testing. As the sun finally went down there was a visual change and a big drop in FPS, which did not recover afterwards.

FPS while sun was still up was approx 85fps, with a frame of around 11ms.

Immediately the sun finally set the FPS dropped to around 40FPs and the frame time jumped up to around 25ms. It stayed like that for some minutes.

Looking at the statistics bar about 5 minutes later the FPS was round 70 FS and frame time around 15ms.

What were you doing when it happened?

Watching the sun set on Bellisseria with Statistics bar open. Using Release candidate 7.1.2.7215179142.

What were you expecting to happen instead?

Frame rate to stay similar in all sun/moon conditions.

Other information

A relog after 5 minutes or so appears to allow the FPS to go back to around 85fps (frame time 11ms or so) rather than the limited FPS it had settled on ahead of the relog.

I had noticed during previous testing related to the NVidia GPU/8 core CPU systems on Firestorm and LL viewers that there could be a big an sudden change in FPS at times when sun was out and not. But the report as shown above was my clearest indication the frame rate drop was exactly as the sun finally left the sky and the sky turned a sudden deeper blue with no sun colour in it.

https://secondlife.canny.io/admin/board/bug-reports/p/pbr-sudden-and-large-loss-of-fps-as-sun-sets