I (Clarke) have a bad habit of trying to read 3,4,5 or more books at one time. My bedside currently has about 12 books, all of which are "in progress". It (unjustifiably, I think) annoys my wife immensely. If A-E represents the 5 books I am currently switching between, and I switch between each every so often then my reading looks like this:

ABCDEABCDEABCDEABCDE ... all finished
                              a finished here
                                b finished here
                                  c finished here
                                    d finished here
                                      e finished here

AAAABBBBCCCCDDDDEEEE
      a finished here
              b finished here
                      c finished here
                              d finished here
                                      e finished here

While it appears that all five tasks finish at the same time. We know from our own reading that it takes awhile to get back into a book. We might have to back up to re-acquaint ourself. And maybe our retention falls off. Projects are just the same. Task completions often release work for another person, consequently multi-tasking significantly delays the release of work and the completion of the project.