Coyle: "This urge for speed makes perfect sense, but it can also create sloppiness, particularly when it comes to hard skills (see Tip #8). We trade precision—and long-term performance—for a temporary thrill. So slow down. Super-slow practice works like a magnifying glass; It lets us sense our errors more clearly, and thus fix them."
Coyle also has a nice post on his blog about called, "Slow is Beautiful," which includes video of golf great Ben Hogan showing how he uses slow practice.
This is a critical rehearsal tool for the conductor, but there are various ways to use it. And surprisingly, I see it used infrequently by too many conductors.
In a fast passage, at a certain speed some members of the choir will never perceive the pitches (or patterns) accurately. You can practice the passage 20 times at a fast tempo and it'll still be sloppy. But a fewer number of repetitions at a slower speed can allow the singers to absorb the pitches and build them in correctly. The same thing is true for instrumentalists. When I rehearse strings, for example, if the music has awkward string crossings, difficult bowings, or simply calls for extreme speed, the only way to make it better is to slow it down. For both singers and instrumentalists, "muscle memory" must be developed that allows passage work that can be done accurately without consciously thinking of every individual note.
With a choir, it also makes it even easier if you take away another variable (text) and sing on a neutral syllable or count-sing (if your choir does that regularly). Since I do lots of baroque music with my chamber choir that specializes in early music, this is always the first tool of choice with a fast melismatic passage or fugue: take away the text and slow it down.
It's the same when performing (or learning) an unfamiliar language. I'll usually do a "repeat after me" session when first going through an unfamiliar text in another language. My going too fast only means that the choir can't even perceive the sounds correctly, much less repeat them accurately—and it will waste time, rather than saving it.
And sometimes to shape a phrase with subtlety, a slower rehearsal speed allows the ensemble to feel and shape phrases together in a way they can't at a faster speed.
Slower can be faster. And slower can much more quickly get the ensemble to a fast, yet clean performance.