As in summary, glxinfo -l reports GL_MAX_VIEWPORT_DIMS = 16384, 16384 which results in too slow to be unusable dual screen composited setup. Original bug report here:
I suspect KDE is using the advertised maximum viewport incorrectly. The OpenGL spec requires the value be fairly large. Section 2.12.1 (Controlling the Viewport) says:
The maximum viewport dimensions must be greater than or equal to
the larger of the visible dimensions of the display being rendered
to (if a display exists), and the largest renderbuffer image which
can be successfully created and attached to a framebuffer object
(see chapter 4).
X allows the user to create very large windows and pixmaps... even larger than the display itself. As a result, we can't advertise a small value here and still comply with the spec.
If KDE wants to know sizes that will be "fast," GL_MAX_RENDERBUFFER_SIZE is likely a better metric.
Unless there is additional feedback, I'm going to close this as WONTFIX.
Looking at the patch in the upstream bug, it looks like they've found the correct solution. Closing.