Graphics Global Illumination horribly slow since 5.3

Since version 5.3, I'm getting horrible performance of real-time Global Illumination.

As soon as I do real-time changes to any parameter of my sun light source (position, direction, intensity, ...) or update my sky box texture, it introduces strong spikes in the frame duration (several hundred milliseconds) that occur about every second.

Unfortunately, this makes the game/simulation completely unplayable with Global Illumination enabled.

Prior to 5.3, everything was running smoothly. I was always using the latest Beta versions.

I attached a screenshot of the Unity Profiler output and the Windows Task Manager. CPU usage is always around 50-60%, no spikes are to be seen there.

Also note that Global Illumination is not showing up in the Unity Profiler, although there is a red "Gi" item in the legend! The spikes we see in the other tasks are most probably the result of the Global Illumination spikes that aren't visible for whatever reason.

Screenshot (click to enlarge):

 

Do you have a bug report with something we can test with?

 

Uploading my project ... will take some time. I will post here when it's done.

 

Done! The ticket ID is: 745096_b7p6cg9mmqtj498i

 

Thanks. Advanced it a bit in our QA pipeline.