It's actually worse than that. There's a lag every time we switch ti a new task/project. One company I worked for experimented with this. We used to "round robin" our projects, to make sure each one had progress. We went to a seminar that encouraged completing one task/project before continuing on to others. At first, it was stressful watching projects go untouched as the deadline approached. However, we were able to complete everything, and with higher quality than before. I think we even had some time to spare, which allowed us to give a little extra and prepare for the next iteration.