Imagine you are driving at full speed on an empty highway and then oops… You are hitting an hour-long traffic jam. Your 8 hours trip becomes 9 hours. How does it feel?
Now imagine that getting back to the full speed takes at least 30 minutes, not ~10 seconds. Right, your trip is now 9 hours and 30+ minutes, 90 minutes longer.
The reason for 30+ minutes is that any intellectually intense work requires flow. Flow is a mental state when the brain is most productive. Getting into the flow state takes time.
Being in the flow is like driving at full speed on an empty highway. Being interrupted is like hitting a traffic jam.
Then you are driving forward and hitting two more 15-minute traffic jams hours apart.
Think you lost another 15+30+15+30=90 minutes? Unfortunately, it is a lot worse, you lost the whole day…
Getting into the flow requires a lot of energy. With every next distraction, it’s a lot harder.
That is why programmers and everyone else who is making great stuff don’t like meetings.