I can't seem to search the Apple support site right now but there is a document that spells it out. The upshot is that journaling requires some drive access and activity to maintain the index in case of a crash. If you are running close to the bandwidth limit of your drive, turning off journaling may give you a bit more overhead. Only recommended for media drives and any drives in which read/write performance is paramount.
That's a rough paraphrase of the apple doc. They do use all the same qualifiers though: may, might, could etc.