Reference #144: Peopleware
Certain types of work — such as engineering, design, or writing — require a state of flow to be done productively. Re-entering flow after an interruption costs a re-immersion time of over 15 minutes.
A worker who is frequently interrupted has a greatly reduced effectiveness. And so the cost of their work increases.
Note this differs from the work of a manager, for whom frequent interruptions is part of the role.
DeMarco, Lister. Peopleware, 2013. (62)