xbel stands for XML Bookmark Exchange Language and .recently-used.xbel file contains list of recently opened files. This file is opened and parsed every time you open a file inside GTK application like eog (Eye of Gnome Image Viewer). This file could grow larger (some times megabytes) and affect the responsiveness of the GTK application. I don't know why GTK team chose XML format rather than something like SQLite which offers faster retrieval and lookup of entries. Assumptions aside lets disable this fracking thing and make your GTK applications much more responsive.
You should have either .gtkrc or .gtkrc-2.0 in your home directory if not create a new file and add following line to it.
Now restart your gnome environment for faster desktop environment.