I learned an early lesson as a musician: timing is critical. This is true when playing in an ensemble or singing in a choir. It is also true when learning and performing music.
Tempo is critical in other areas, serious bridge, for example. When I play duplicate bridge, I have to be aware of the tempo of my bidding and play, lest I give my partner a hint of unauthorized information she or he would not be entitled to were we playing fairly. Example: if I take a long time to make a play, my partner might presume that I had a tough choice and alter his or her strategy accordingly. It is a violation of ethics to intentionally communicate to one’s partner one is having difficulty in this way. And when my partner disrupts playing tempo, I’m obliged to bend over backwards to ignore the delay and play as normally as I would had the play or call been routine in tempo. On a more informal game-playing level, players who take a long time to make bids, or place bets, or think too long, disrupt the tempo of a game to the detriment of others’ pleasure. (Don’t take so long to consider buying Illinois Avenue; just fork over the dern $240!)
My tradition-advocating church music colleagues wring their hands over the lack of hold plainsong has in American parishes. I suspect the number one musical reason is tempo. I remember an otherwise excellent musician setting a tempo for “O Come O Come Emmanuel.” No wonder her choir turned up their noses at it. It was mournfully slow (quarter=64 or 68). My preference is about 112-120, so you can see the difference of opinion we had about the hymn. If Catholics are exposed to chant thinking it is meant to be performed slowly, their teachers and music directors have poisoned the waters.
Tempo is critical for all kinds of hymnody. Musicians must be able to understand the music they play and accurately play the tempo the composer has set. Even then, judgment is required. I think some music sings and plays better at slightly faster tempos than composers have written or recorded. Some of them seem to think so, too; when I go to concerts, inevitably, the tempo is faster on most songs than what I’ve heard on recordings or recall from the scores. Playing too fast is a danger, too: it is easy to speed the life out of a song just because you know it very well.
If you are trying to incorporate chant into your parish repertoire, be advised the music is more demanding than it seems at first glance. It might seem to be simple enough: a single line of melody, but there is a zen-like quality that will unfold the experience as you properly engage the text and music.