[sanderjd]

They might be "parallel" if there were no zero-sum time trade off, or no time pressure. I wouldn't mind so much spending time fielding interrupts all day every day, if I didn't have anything else I was trying to get done. But that's not the reality. And for me (and I think many others), it isn't just additive, because time pressure makes me anxious and less focused, which slows me down more, which increases the time pressure, which makes me more anxious, etc.

By the way, I'm not trying to say that this is always happening in my work, or even frequently, but this is an actual bad outcome that I experience from time to time. I currently have, and have been fortunate in my career to usually have very good management and teammates who are very respectful of each other's focus time.