When you log into the VAX, you get a start-up message saying something
like This is VMS version 5.1, together with a lot of extraneous
garbage. Modern multi-user computers all come with a master control
program or operating system which sits in the machine continuously
looking after all the other programs. On the VAX, this program is called
VMS.
Generally, such computers are designed so that they can only run one program at a time. To give the appearance of dealing simultaneously with the people in this class plus all the other users elsewhere, the operating system time-shares. It picks one terminal, sees which program is to be run there, and runs it for a few milliseconds. It then interrupts this, saving the state the program had got to, and picks another terminal. It runs that program for a few milliseconds, interrupts it, saves it, and picks yet another terminal. Eventually, it gets back to the first. If each program is only run for a very short time on each round, you get an effect like that of film frames, where the operating system comes back to you so frequently that your program appears to be running all the time. This resembles some theories of attention in psychology, and indeed they were probably inspired by this idea of a master controller sharing out resources amongst lots of subordinate programs.
If there are many other users, the operating system can devote less proportion of its time to each, so the programs appear to be running slower. This happens especially in mid-morning and mid-afternoon, and accounts for all those irritating pauses.