Why would the frame rate double when detached from the player controller ?

Development Rendering
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1

I upgraded to 5.1 this week, and decided to build up the game world from scratch to understand the performance impact of everything.

I started with an empty world outliner, and added just a directional (sun) light, sky atmosphere and sky light. I also added a static mesh that will act as terrain.

I set the game mode to BP_ThirdPersonGameMode in the world settings. The editor viewport is maximized to about 4k resolution allowing for the toolbars.

I press ‘Play this level in the active level editor viewport’ and the character drops onto the mesh, and I can look around and move around normally.

The frame rate is around 40 fps and, when I use ProfileGPU, the bulk of the frame render time is ‘DiffuseIndirectAndAO → LumenScreenProbeGather’ that takes 17 ms

When I press ‘Detaches from the player controller, allowing regular editor controls’ then keyboard and mouse control is returned and the game looks exactly the same

The frame rate however has doubled to around 80 fps and, when I use ProfileGPU, the bulk is once again ‘DiffuseIndirectAndAO → LumenScreenProbeGather’ but it only takes 3.5 ms

Why would there be so much discrepancy in Lumen; maybe the camera setup is different between the attached and detached mode ?

2

I have found out that the ‘Screen Percentage’ when set to 51% as the default, will cause the above phenomenon.

When I set the the Screen Percentage to 100% then the frame rate is around 40 fps when either detached or attached.

It seems that the Screen Percentage only pertains to the viewport when detached from the player controller, but otherwise doesn’t affect anything else ?

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