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So I have a laptop with 2 GPUs: an Intel HD 520 and a Geforce 940MX.

In the case of 2.79, if I had set Blender to run on the nVidia GPU in the NV control panel, there was a horrible GUI lag when there was rendering in progress using Cycles. All GUI elements took 2-3 seconds to react to mouse hover and mouse clicks. This is completely normal, of course, because the GPU that is used to render the GUI is also being used to render the image in Cycles. And of course with CPU rendering, this GUI lag didn't happen.

If I had set 2.79 to use the Integrated Intel HD GPU, there was no GUI lag because only Cycles render engine used the nVidia GPU and the rest of Blender used the Intel HD GPU.

Now, in 2.80 I am having the same issue, but this time, configuring in the NV control panel to run Blender on the Intel GPU doesn't help. The GUI still lags like hell, and it's almost unusable because I can't make fast changes, tinker with materials etc. while it's rendering. (In 2.80 CPU rendering still works fine as expected.)

  • Same thing with GPU and CPU+GPU rendering (GPU Compute).

  • I have Geforce 436.30 drivers (most recent).

  • 940MX is set to 'Maximum Performance' in NV Control Panel, other settings are default.

  • Windows 10 v1903.

  • Blender 2.80 most recent stable version.

Was there a change in how the GUI gets rendered in 2.8 or something? If anyone can help, I would really appreciate.

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  • $\begingroup$ Integrated graphics card are basically useless... In any case, open up the task manager and see how your resources are distributed. While editing in Blender, is the CPU to be hit the most, except when you have viewports in rendered mode (my GPU is always at 0% load when not rendering) $\endgroup$ – Tareyes Sep 28 at 18:36
  • $\begingroup$ @Tareyes Integrated GPUs are useless for rendering, but they are good for rendering the interface of Blender so that it stays responsive instead of responding every 2-3 seconds. At least if that's what happened in 2.79. There was no GUI lag during rendering when Blender was set to run on Intel GPU (and the 940MX was at 100% constantly). $\endgroup$ – Lajos Nagy Sep 28 at 18:46
  • $\begingroup$ Anyway, right now during viewport preview render and GPU+CPU rendering, 940MX is at 100%, Intel HD is at 5%, CPU at 70%. Around the same with only GPU rendering. $\endgroup$ – Lajos Nagy Sep 28 at 18:48
  • $\begingroup$ I know it might not be what you want, but have you considered rendering in GUI-less mode? It's quite faster than the normal one, and since it's 2 different processes, you could remove any multithread incompatibilities $\endgroup$ – Tareyes Sep 28 at 19:41
  • $\begingroup$ @Tareyes I've considered it, but it would be kind of useless to me, since I have no problems with rendering the final image, because I don't use Blender during that. Blender needs to be responsive during rendering the viewport rendering process, and that workflow is tied to the GUI. $\endgroup$ – Lajos Nagy Sep 28 at 21:04
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As a workaround, I am now using Blender 2.80 on a Kubuntu 18.04.3 system. The interface, while not perfect, works much smoother than on Windows (while rendering viewport view).

Since the nVidia driver doesn't get loaded on Kubuntu if the Intel GPU is selected (sudo prime-select intel), I am forced to use the nVidia GPU as primary, and so Blender uses it for both rendering of interface and rendering in Cycles. But this is still so much smoother than on Windows 10. The GUI elements react quickly to mouse interactions.

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