Shutter lag (the time between pressing the shutter button and when the camera takes the picture) is caused by electronic systems completing check before making the exposure, by a zoom lens adjusting the final focus, or by the time needed for simple mechanical movements. Pro DSLRs have minimal shutter lag (15 milliseconds is good) but lower-end cameras may be 0.2 seconds or longer.

You can reduce shutter lag by shutting down as many automatic functions as possible.

Adapted from Fundamentals of Photography by Tom Ang (Alfred A. Knopf, 2008;$25)