Saving a figure from the GUI under GTK triggers a lot of IO activity. Up to 60Mb/sec on my system.
The file /.local/share/recently-used.xbel gets written to each time the session history is scrolled through with cursor keys.

I am running matplotlib 1.2.0 but this issue is, I believe, much older than that.

I reported the issue to IPython (see ipython/ipython#2608) first thinking this was a purely IPy problem but I could then reproduce the bug in a normal python session.

As suggested by rkern there, I posted a dump of strace and lsof at http://filebin.ca/NdspC6wfTV3

Another user also reported that saving from script triggers the same problem, but I could personnally not observe that with certainty.