(In reply to Darek Deoniziak from comment #18)
> First made only intel use DRI3 and radeon DRI2 - that felt like when I set
> both GPUs to use DRI3 (same issues).
> 
> Second time I tried DRI3 for radeon and DRI2 for intel (running it
> currently). It feels like when using DRI2 for both, [...]

Your feeling is correct: The secondary GPU's Xorg driver configuration doesn't
matter for this.

Your remaining issues are mostly related to lack of synchronization between the
GPUs. This should be better with the i915 driver in recent kernel versions
(might require 4.9-rc or newer), see bug 95472.