Hello,
I created an account because 2 days ago I was having the same issue and wanted
to provide my dmesg. But when I was trying to reproduce the issue yesterday, I
couldn't... I played for hours and the performance was actually quite good.
I am with fedora 22 on KDE 5 now, and I did perform some updates before trying
again... The other thing I did was disable the option to suspend compositing
for full screen windows. Not sure what fixed it though