This is the results of my findings after thorough testing of each individual slider in P3D, in case you’ve wondered which settings affect CPU usage and which ones affect GPU usage.

 

CPU: 

Water quality and all water reflections 

Texture resolution, mesh resolution, LOD radius 

Special effects (detail and distance)

Scenery complexity, autogen draw distance, vegetation and building density, dynamic 3D vegetation

Cloud layers, density and draw distance 

All types of AI traffic 

 

GPU:

Anything to do with shadows and light (quality, distance, boxes ticked, dynamic lighting and reflections) 

Tesselation 

Anti-aliasing 

Texture filtering 

Screen resolution 

 

 

There's too much interaction between various things to make a flat statement like that. Interesting observations but valid only on your system.

Consider, there are some functions that can overload the GPU - when that happens, the CPU is affected and the FPS start to drop.

Vic

The Avsim P3D guide provides similar and more extensive information about CPU/GPU usage:

 

It can depend on your CPU/GPU ... for example Water = Ultra and High will use nVidia CUDA library which should make most of the work happen on the GPU, this was recently updated to work better with 10 series nVidia GPUs.  However, if your GPU isn't CUDA capable there is the possibility it will fall back to software rendering on the CPU.

Reflections should be mostly a GPU process as are shadows, but again, if for some reason the GPU is either being overloaded or a shift from hardware to software rendering, you could see an impact in FPS.

There are other settings also and as Vic has pointed out overloading the GPU ... running out of VRAM will trigger memory swapping which will slow down FPS considerably (this can happen with TEXTURE_SIZE_EXP=10 or even 9).

For things like clouds, it really depends on what types of clouds, 3rd party or default, and if 3rd party what texture sizes are being used for the clouds and what AA settings are being used and if any 3rd party product is changing cloud size ... these will have a very significant impact on FPS.

For AG, Tree density (non-speedtrees) has little to no impact on my system, but AG buildings do especially in combination with building shadows cast/receive.

Special effects is a bit of a mystery to me, in some areas it can have an impact to FPS, in other areas not, as a rule of thumb I've been turning these settings down to lowest as they seem to add little to my visual experience and they don't seem to impact custom 3rd party special effects.

On a positive note, a recent public announcement by Lenovo regarding their new laptops and desktop PCs will be using nVidia 1160 GPU and 1180 GPU ... unintentional or intentional leak, who knows, but that suggests new GPUs from nVidia might be coming very soon (August).

Cheers, Rob.

1 hour ago, Rob Ainscough said:

For AG, Tree density (non-speedtrees) has little to no impact on my system, but AG buildings do especially in combination with building shadows cast/receive.



Have you noticed any impact with cast/receive shadow settings for trees (non-speedtrees) or is the comment you stated just related to density?

19 hours ago, MikeT707 said:

Have you noticed any impact with cast/receive shadow settings for trees (non-speedtrees) or is the comment you stated just related to density?

Density only, I don't like how shadows render for the X Trees hence don't enable AG Tree shadows.  Because the X Tree is just two planes the shadows for a X just don't render well at most viewing angles (with a few exceptions), not to mention there can also be PR ground shadows baked into the terrain tile which results in a X shadow from the East and baked in shadows from the South (for example) ... two different shadows, but one source.

Cheers, Rob.