Downstream bug: https://bugzilla.redhat.com/show_bug.cgi?id=835740 (and before it: https://bugzilla.redhat.com/show_bug.cgi?id=816764 ) Unchecking the 'Use slow keys' box in accessability prefs doesn't seem to disable it. Holding down a key for 10 seconds enables it. I don't know if the X setting to disable this changed or what. Happy to gather more info, etc.
This is because the dialog just updates the Xfconf properties which are only read when launching xfsettingsd. There is currently no channel monitoring for those properties. Thus, at the moment, it's more "Enable **** on startup".
Sadly it also seems to be a gdm bug where it turns it on somehow and xfsettingsd can't/doesn't unset it. Using lightdm/etc it works as you note to enable/disable on boot.
That was a bug in gdm / gnome-settings-daemon which is now fixed upstream: https://bugzilla.gnome.org/show_bug.cgi?id=685063.