IIRC diagonal lines are not accelerated with glamor right now (so SW rendering would be faster) but there are some initial patches to accelerate them. I think you'll find you're running with SW rendering right now so you'll never be happy with the results -- suggest you go back to glamor but hunt down the patches to speed up line drawing. If I find the thread / bug ticket I'll post it back here.
No announcement yet.
AMD RadeonSI Driver 2D Performance Is Getting Better