I still can't repro the issue. Another difference might be that I'm using xorg
1.13, whereas your original log is for 1.12. Is there anything in dmesg when
the hangs occur?

I did notice that if I'm running with a compositor, e.g. xcompmgr, then things
get REALLY slow when loading large images. Slow enough for watchdogs to fire,
rcu to complain, etc. But the system does come into its own, esp if I kill
xcompmgr. I don't know whether "standard KDE settings" include a compositor
(wouldn't be surprised), but if they do, can you try turning that off?