A typical tempo for a rock song is 120 beats per minute, in the standard metronomic scheme. If you’re used to a song at 120 you can feel the difference if it’s played at 122. It sounds a bit caffeinated. At 118, it’s dragging a little.
I know from performance experience that when a band launches a song in the wrong tempo the musicians have only three choices, two of them unsavory: (1) endure the nauseating peppiness or dragginess of the wrong tempo, (2) pull everyone else up or down to the right tempo, hopefully within just a few measures, as unobtrusively as possible (not easy to do, not what you want to be concentrating on), or (3) find a hitherto unknown, different feel of the song corresponding to the different tempo. #3 is a nice result when you can get it, but by no means guaranteed to be available.
As listeners, especially to recorded music, we mostly benefit from good aesthetic calculations and competent performance tempo-wise, and this basically means that tempo goes unheard. A cover version can give you a chance to consider the merits of an alternative tempo for a song you know, but usually the covering band is doing a bunch of other things differently as well, giving the whole performance its own Gestalt; rock listeners don’t have to suffer the tortures of Mozart fans being subjected to “historically correct” speeded-up or “Romantically” slowed-down performances of the pieces they adore. Still, sometimes a rock cover comes out that doesn’t make enough good changes, and you can tell the tempo sucks. When the Police covered their own boppy hit “Don’t Stand So Close” at a much slower tempo several years later, it was excruciating.
When was the last time your main rave about a song was: “Killer tempo!”? I know it’s just too music-geeky to say such a thing, but in one case I have to. It’s Led Zeppelin’s “Since I’ve Been Loving You,” already pretty great at a super-stately 40 beats per minute on Led Zeppelin III but utterly majestic at around 38 bpm in their 1972 Long Beach Arena performance. Clearly they were trying to get tempo-radical: the slowest they’d dared to go in their earlier recordings was 52 bpm in “You Shook Me.”
Explaining killer tempo is no slight challenge. A normal good tempo just sits in between too fast and too slow without a positive quality of its own. Obviously a good tempo will allow the various sounds and events of the song to get well articulated. Obviously it will also have some relation to our self-articulation, our pulse or our breathing or walking or dancing rate; it will be nipping after us or settling us down. Okay, so what else?
There must be something improbable about a killer tempo. It must be in tension with an expectation we have, such that we’re sold on the divergence—rather like a flatted blue note presses down on the scale’s normal pitch and makes us like it. It tells us “We can do this,” and we exult in success with it. The moderately slow killer tempo of “Hold On, I’m Comin’” (compare “Knock On Wood”) shows us how we can we sashay grandly and shake our tail feathers at the same time.
In the extreme slow killer tempo of “Since I’ve Been Loving You,” what we find we can do is stew in the juice of each measure without getting bored or distracted. (It’s important that the chord progression of the song is a little unusual and quite lovely: we always have something to look forward to, but we’re never being pulled forward too strongly.) The slowing-down, sinking (think of a weighted diver entering deeper blue water), concentrating power of the riff is stronger than the default upward pressure of a normal, brisker tempo. This sinking dilates experience itself (raptures of the deep!). In this particular song, slowing the tempo down has the same effect as turning up the guitar amps: a more intense edge, more emanations, more overtones and feedback. On the plane of the vocals, Robert Plant’s chattering of blues phrases proves that the slow tempo of the song is more stimulating than stabilizing. Jimmy Page’s guitar solos do the same. But the longer-than-expected intervals between the main beats sustain a strikingly open time-space for events that feels like it’s always expanding, inviting us interestingly (instead of shockingly as a fast song would) to hold on to the song’s pulse ever more firmly while letting us feel the proceedings ever more fully. The whole song is a long journey—more than eight minutes in the Long Beach Arena performance—and that is a big opening for experience in itself, but the quality I’m talking about is in every measure, once the song gets going.
The slow killer tempo evokes worlds of experience for which it has made time. What, then, is an example of a fast killer tempo, a constantly blasting-off tempo that strips us free from experience-luggage we don’t want or need?
 According to a recent study in Scientific American, the average tempo of pop songs has gone down from 116 in the 60’s to 100 in the last decade. So moody have we become!
 Lynn Raley has chided me for my poke at historically informed retrieval of the true quality of Mozart’s music, which I should learn to appreciate less in terms of singing and more in terms of speaking. What can I say? My Mozart sings.
 Actually, two cases, but I want to write about Al Green’s “Here I Am (Come and Take Me)” on a different point.
 The 1972 Long Beach Arena performance appeared on a CD, How the West Was Won, in 2003. A 1973 Madison Square Garden performance featured on the Led Zeppelin DVD of 2003 also starts around 38 bpm but flexes the tempo more dynamically and pushes it down even lower in the end. “Dazed and Confused” could be taken very slow too, about 41 bpm on How the West Was Won. As the Professor of Pop Andrew Goodwin has noted, Led Zeppelin’s tempo is characteristically mobile, so all these fixed figures are misleading.