I too just came across the problem with kio_thumbnail. There were upto 30 instances of it running consuming upto 70% of my CPU.
I found the problem was folders that had a mix of video and images and the thumbnailer trying to decide what thumbnails to use for that folder with the mix.
I deselected 'Directories in Dolphin via Settings > Dolphin Preferences . General > Previews. Also i swapped the video previewer from Video Files (kffmpegthumbnailer) to Video Files (MPlayerThumbs) and now my system doesn't come to a grinding halt with 30 instances of kio_thumbnailer open and I can navigate through large folders with video and images without a drama.
that doesn't seem to solve the problem, any other solution ?