xfce4-settings-helper's clipboard manager seems to be eagerly making copies of clipboard contents. This causes quite a bit of traffic, especially visible with a client that can intentionally rapidly mutate clipboard, like emacs. See emacs-devel thread at [1] for more discussion.

The overhead would be fairly unsurprising for a full history-keeping clipboard manager, but is heavy for a minimal default one as I imagine the xfce4 one is intended to be.  The behaviour is in contrast to the small clipboard manager in gnome's gnome-settings-daemon, that clients can ask for their clipboards to be saved at exit via the fd.o spec [2] SAVE_TARGETS mechanism - that manager does not eagerly copy the full clipboard contents from the owner each time.

So could xfce4-settings-helper's clipboard manager be made less eager? Either for all clients, or at least for clients showing the SAVE_TARGETS "marker target" mentioned in the spec - it can be assumed that at least those clients do know to ask if they want the clipboard contents saved on exit if they exit while owning the clipboard. (in the latter case, then at least once emacs is actually patched to follow the spec, the overhead wouldn't be there).

[1] http://lists.gnu.org/archive/html/emacs-devel/2011-05/msg00296.html
[2] http://www.freedesktop.org/wiki/ClipboardManager