When printers had their type set by hand, they would pay their employees by piecework. Work product would be measured in ems. The faster you worked the more you’d be paid, though of course speed without accuracy would not do, as the corrections you’d need to make would have to be done before you were paid. “Ordinarily, in fact, printers simply wound a piece of string around the block of type they had set, measured it, and were paid accordingly. A fast printer set a ‘long string’.”*
In a ten-hour day the average journeyman would set (and correct) about 7,000 ems, 700 an hour. At a rate of 1,500 ems an hour, which most compositors would achieve in spurts, their hand would be reaching into the typecase at a rate of 4,000 times per hour. Very fast workers might reach back and forth from case to stick seven or eight times every five seconds.
* Walker Rumble: The Swifts: Printers in the Age of Typesetting Races, University of Virginia Press, 2003