Sound is one of the late senses. Only smell is slower, but its reactions often have a primal immediacy that belies its leisurely spread. Touch and taste are obviously close at hand (or tongue), and vision arrives with the speed of light. But sound takes its sweet time sauntering along, maybe stopping to radiate and reflect before it finally shows up, unashamed, and monopolizes the bean dip.
I remember that the first time I'd really seen with my own eyes how slowly sound travels (relatively speaking, of course) was during a summer camp in Indiana, watching kids in the batting cages from a half-mile off. It was close enough to see a batter hit the ball, but the crack of the bat wouldn't arrive for another half-second. Most of the time, we don't really pay attention to the lag involved in travelling sound waves, because the distances for our interactions are so short. But sometimes--say at that park, or when the drone of an airplane lags behind it in the sky--we can't help but notice.
Of course, no matter how tardy everyday sound can be, it only gets worse when you run it through a computer. The lag between input and output on a computer audio interface is called latency, and it's measured in milliseconds. Digital audio enthusiasts trade latency numbers the way car enthusiasts swap gear ratios. The lower latency, the more responsive the interface can be, and the less it will throw off a musician's timing. The closest analogy I can think of, for non-musicians, is actually playing a video game online, where the inputs are delayed slightly (I think they compensate for this on the client-side now, but I remember it from Quake). With small amounts of lag, the player might not even notice. As the lag increases, the game starts to feel a little "floaty" and players have to mentally compensate. When the lag becomes too high, keypresses become disconnected from the action onscreen, and it's impossible to play effectively. The amount of latency that can be tolerated in a game or a DAW varies from person to person, and musically it also depends on the instrument: some instruments have almost no natural latency (drums), while others involve physical mechanisms or note "bloom" (piano, bass guitar) and their players tend toward greater tolerance.
My Tascam US-122, one of the first USB audio interfaces, has pretty terrible latency. That's not why I'm selling it, but it's been noticeable and much more than newer interfaces. I get just under 30ms, compared to reports of less than 8ms with the Line 6 Toneports. 30ms is still not very much--.03 seconds---and it's enough to throw me off when I'm singing, although I don't notice it at all when playing bass. Like all modern computer soundcards, you can route the signal directly through to the outputs for almost-zero latency, but then you don't get the effects processing, and that's one of the main reasons I work with computer audio in the first place.
With all that said, I want to address a misconception that I hear from a lot of musicians with regard to computer-based effects. Many people say that even the slightest latencies throw them off. I've heard people say that even a 6ms delay is enough to disrupt the groove's timing. And frankly, it's all in their head. It's a form of snobbery to be able to say that you can hear such a short delay, one aimed at people like me who can comfortably play at much higher latencies (must be something wrong with us!). Here's why:
Remember the batting cages? As that example shows, sound moves through the air relatively slowly compared to the speed of light. In fact, when you work out the math, it travels about a foot for each millisecond. This is the physical latency of sound--for each foot from your amplifier or sound source, add a millisecond of latency. In other words, someone who complains about six milliseconds of latency is basically claiming that if they moved six feet further away from their amplifier, they'd be unable to play. Given the number of wireless systems and long cables typically used by rock musicians, I doubt that they could actually tell the difference in a blind test. It just sounds cool to say that your rhythm is so tight that even a .006 second difference throws it off.
Latency--physical or digital--is simply a natural part of musical performance. In an orchestra, performers need the conductor because they might be 40ms of distance apart. From the audience's perspective, they are all in time, but adjustments must be made from instrument to instrument in order to preserve that perspective. But musicians are nothing if not distrustful of technology, a perspective that Autotune and Pro Tools have done nothing to change. Digital latency is just an easy boogeyman. The next time someone makes a claim about their delicate timing, you might ask them to literally take a step back from that opinion.