If, however, one has decided that one isn't going to think about the employee performance reviews that are due in 3 months because one has to finish up a programming task first, that's true in a sense of *priority* - but it also misses the potential opportunities to move "employee reviews" forward with phone calls, material review, and dozens of other little next actions that might be able to happen relatively seamlessly in the meantime *without* sacrificing any progress on the programming task.
But my view--and of course no one has to agree with me--is that you are indeed sacrificing progress on the other projects. My view is that every added project hampers every other project, merely by reducing focus.
That doesn't change the fact that having multiple parallel projects is often, usually, unavoidable. But I do argue that when it is avoidable--and IMO you absolutely could avoid those employee reviews for two and a half months--I think it's best to avoid.
Imagine that you're in the middle of hunting down a code bug. You've got six windows open, you have five methods and four variables and three theories spinning in your mind. You've implemented the code for a theory, and let's pretend it's 1982 and the resulting compile is going to take ten minutes.
During that ten minutes, you could sit there with your mind mostly idle, though likely absently musing over the problem, OR you could make an intense focused phone call about a detailed and emotional issue unrelated to the bug.
My view is that sitting there with your mind idle, "wasting" those ten minutes, will leave you much more effective when the compile is done and you're ready to resume the coding. I think, in fact, that making that phone call will cost you a great deal more than the wasted ten minutes.
Now, that's a task switching example. It might seem totally irrelevant to a situation where you spend four hours on Project A, leave that task to go to lunch, return from lunch to spend ten minutes on Project B, fifteen on Project C, ten on Project D, and then return to Project A.
But I don't think it is irrelevant. I think that B, C, and D are harming A. If B, C, and D are unavoidable, then after lunch (or the next morning) is indeed probably the best time to touch those projects, all in a group, so that A gets several unbroken hours again. But if B and C could be moved to Friday, a day full of meetings anyway, and D could be moved out a month, that is IMO better.
And while my example is programming, I'm not just thinking about programming. As another example, I've been trying to finish the first draft of my novel, and as it gets more difficult--as more of the plot is laid down, and the plot holes and character questions are harder to resolve and concepts are harder to express gracefully--I've been finding that kicking other projects out of my mind greatly increases my progress. I don't think that this would have been so clear to me if I hadn't been trying to do something that is close to the limit of what I'm capable of, so that I'm looking at making meaningful progress versus making zero progress. But I also don't believe that the effect is limited to those "personal limit" situations.
I mean, you have to have something to do when you're sitting there twiddling your thumbs while you're waiting for your meetings to start, right?
In my case, no. I am increasingly, deliberately, by design, doing nothing during those idle times.
I remember reading an article about the value of boredom. I'm going to try to find it.