Comment 4 for bug 733425

That's it Didier!

A mouse-motion event received through the X11 connection while dragging calls
  Launcher::UpdateDragWindowPosition ()
which calls
  LauncherModel::ReorderBefore () or LauncherModel::ReorderSmart ()
which calls
  LauncherModel::Sort ()
which emits
  LauncherModel::order_changed
which calls
  LauncherController::SortAndSave ()
which calls
  FavoriteStoreGSettings::SetFavorites ()
which calls
  g_settings_set_strv ()

That function from libgio calls the write vfunc from GSettingsBackendClass. Commenting that call removes the I/O peak and makes the rendering faster.

I'm not sure why the favorites must be written while dragging, but I guess it could be done at the end of the drag?