I just noticed that on my newly setup machine (Ubuntu 15.10) the "accounts-daemon" process was using up nearly 100% cpu time.
After some googling i found that it was because /var/log/wtmp had grown to over 600MB in size. Since that is a binary file i can't read much out of it, but it contains lots of words like tmux, 'my_username', pts/7, pts/16, pts/18 and so on.
Does anyone know what this is about?