So, I was able to give it a try... but bad luck, it does not work, apparently because the shader is not working with compatibility renderer.
I have this error message at startup :
SHADER ERROR: 'hint_normal_roughness_texture' is only available when using the Forward+ renderer.
and when I open a document, the view is blinking, mostly showing random white shapes that take most of the canvas.
So I'm afraid that falling back to compatibility renderer will require a bit more work ^^'
I'll try to do some experiments with Forward+ renderer on my Godot project, see if I can reproduce the issue, I still find it strange that it would cause such lags... I should also be able to test my pixelover file on another machine, to see if the results are different.
If you have other ideas or tests that I can run, let me know ;)