Earlier quoted context omitted.
"The cost of an interruption is not just the time it takes, but that it breaks the time on either side in half. You probably only have to interrupt someone a couple times a day before they're unable to work on hard problems at all." -- PG
I (of course) think of it like multi-tasking in a computer. The machine needs to capture the context (stack and registers) associated with one task in order to switch to another. However, for a human this context can be enormous. Depending on the task, it is possible that one will never get back to the state of understanding that they had at an earlier point if working on a particularly complex thing - just the right…
I eventually got into 'the zone' and I was actually quite good at figuring out problems with little context, but my gumption was gone nevertheless, and I was useless for whatever thing I was working on before the interruption for mostly the rest of the day.