[vtkusers] Offscreen rendering speed VTK 8.1.0 + Mesa 13.3.3 on headless VM

[Sophonet]

Hi list,

for a few weeks, I am experimenting with Offscreen rendering using VTK 
8.1.0 and Mesa 13.3.3 on a headless VM (Intel Pentium Broadwell, 2.4 
GHz, one core available), and I am wondering what rendering speed would 
be possible for a 1700x900 renderwindow, when displaying a single 
vtkImageSlice covering more or less the complete viewport. On my system, 
rendering of a single image using the WindowToImage filter (RGB, 
transparency turned off) needs about 300~400 ms, which is a little slow 
in my opinion. Is there any way to increase speed or is this a realistic 
value?

I have compiled Mesa 13.3.3 myself according to the settings in 
https://www.paraview.org/Wiki/ParaView/ParaView_And_Mesa_3D.

What would be the best options for increasing performance (besides 
making the renderwindow size smaller)? Exposing more cores to the VM? 
Some environment variables? (I have played with LP_NUM_THREADS but this 
did not have an effect). Or according to other experiences, there might 
be something misconfigured on our system?

Thanks,

     Sophonet