Starbucks Corp. is telling its harried baristas to slow down—which may result in longer lines.

Amid customer complaints that the Seattle-based coffee chain has reduced the fine art of coffee making to a mechanized process with all the romance of an assembly line, Starbucks baristas are being told to stop making multiple drinks at the same time and focus instead on no more than two drinks at a time—starting a second one while finishing the first, according to company documents reviewed recently by The Wall Street Journal.

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