Usually when I slurp a file, it's not for speed. It's because the file format is easier to deal with if I grab it all at once. For instance, right now I'm building a module that has a fairly complex mini-language for its configuration file. I'm certain there won't be any speed benefit to slurping this file (a typical application will read the config exactly once at startup), but it's easier to feed Parse::RecDescent data that way.
"There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.