Comment # 9 on bug 1135237 from Peter S�tterlin

(In reply to Stefan Dirsch from comment #8)
> Another idea, maybe you're falling back to Software Rendering. Could you
> provide the output of
> 
>   glxinfo | grep 'OpenGL render'
> 
> when using modesetting driver? Maybe the appropriate Mesa package is missing.

No, that seems fine:
astro@astro1:~> grep 'OpenGL render' glxinfo.*
glxinfo.intel:OpenGL renderer string: Mesa DRI Intel(R) UHD Graphics 600
(Geminilake 2x6) 
glxinfo.modeset:OpenGL renderer string: Mesa DRI Intel(R) UHD Graphics 600
(Geminilake 2x6) 

I agree that my special case can be solved differently, still, connecting to a
running X server via x11vnc is a quite standard use case, though I agree it's
low priority.

I just remembered I have some other intel graphics computers around here,
Shuttle with HD5500:
OpenGL renderer string: Mesa DRI Intel(R) HD Graphics 5500 (Broadwell GT2) 
OS is Leap 42.3. It's also running modesetting, but does not show the lag.  So
it might really be linked to the support of Gemini Lake..