Sometimes hibernation slows the PC only when you are working with heavy apps. or games.
e.g. if i'm playing NFS MW (on 512MB RAM pc), after resuming from hibernation (game's not exited, just minimized), the game runs DAMN SLOW, with horrible frame rate issue., the game returns to normal speed after i restart it. (not pc), & sometimes the RAM gets full becoz of a app. (Opera, IE) or faulty app., so to recover the normal PC speed, you got to restart the pc.