After update to Kubuntu 17.10 video playback is buggy. On most video files Gwenview brings the whole system to a halt, just leaves some seconds to interact and kill the application to rescue the system or a warm reboot is necessary. This happens elsewhere as well e.g. previewing video in Dolphin. In contrast VLC plays the videos smoothly and the system functions as usual. I guess some updated backends are at fault. I opted to file the bug under Gwenview since it was not readily obvious where is the fault.
I could not get a freeze to happen on my box , a high end system76 Bonox8 But I did see that on Kubuntu 17.10 both Dolphin and Gwenview are using very high amount of CPU cycles ,, 45-50% as opposed to 3-8% in Neon.
This is probably caused by issues with GStreamer. I would recommend filing a bug on Launchpad: https://launchpad.net/gstreamer