So I removed those files owned by root.
I thought at first it had fixed it.
Turns out, no, it was just taking a while to start hammering.
I made a diff of lsof output for mate-settings-daemon before and during the runaway:
Then I took a peek at that directory, turns out it was host to a truckload of thumbnails that were produced as a result of the step to reproduce I made for the memory leak bug.
ouroumov@Bloc:~/.cache/thumbnails$ ls normal/ | wc
41442 41442 1533354
Those were all thumbnails for the images produced:
Apparently mate-settings-daemon is listing this directory on startup, and there does not appear to be a size limit.
@Wimpy, could a max number of thumbnails settings be added somewhere, or an arbitrary limit be enforced?
I have now rebooted, and I can confirm my system is back to his snappy self.
Thanks