It is considered most efficient to do one thing at a time or wait on one person at a time. As far as possible, you shouldn't let circumstances, unforeseen events, interfere with your plans. Interruptions are a nuisance. Time is limitless and not quantifiable. There is always more time, and people are never too busy.
Time is the servant and tool of people and is adjusted to suit the needs of people.
Schedules and deadlines often get changed. People may have to do several things simultaneously, as required by circumstances. It's not necessary to finish one thing before starting another, nor to finish your business with one person before starting with another. You always have to take circumstances into account and make adjustments. Strictly speaking, there so such thing as an interruption.