In Xubuntu 9.10 (at least), if you open an image in Ristretto the default configuration allows the application to load thumbnails of other images in the same folder.
This is handy for browsing the folder, but it also slows down the image viewer. The current Preferences dialog box for Ristretto doesn't let you change this behaviour. You can turn off thumbnails in a Slideshow, but not in simple Display.
To get Ristretto to open without thumbnailing, do a simple edit of the ristrettorc file, which (in Xubuntu, at least) lives in /home/[username]/.config/ristretto and doesn't need root privileges for editing.
Open ristrettorc in a text editor and change the value of ShowThumbnailViewer from 'true' to 'false', save and close. You may need to log out and back in to get ristrettorc to reload. (If your text editor is Gedit (2.28.0), changing ristrettorc will automatically launch a reload dialog.)
Ristretto will now work molto più rapidamente!
Whoops! Thanks, etnlWings, my mistake. Shows I look first for a configuration file before checking menus...