Lewis and Ribisi try out various intellectual ranges — from Light Gump to Extreme Rain Man — as mentally challenged lovers struggling for independent lives. Diane Keaton’s job as Lewis’ loving but overprotective WASP mom is, of course, to thwart them. There are the usual Garry Marshall directorial distractions (peppy, go-get-’em songs; a passel of epiphanies), and some new ones (subplots involving marching bands and lesbians). Still, there’s no mistaking the central message: Slow people have much to teach us. Or is it: Slow people — aren’t they funny? Either way, it’s pretty vile stuff.