Why does my program run faster if I click and hold the caption bar?

Sometimes, people discover that a long-running task runs faster if you hold down the mouse. How can that be?

This strange state of affairs typically results when a program is spending too much time updating its progress status and not enough time actually doing work. (In other words, the programmer messed up badly.) When you click and hold the mouse over the caption bar, the window manager waits for the next mouse message so it can determine whether you are clicking on the caption or attempting to drag. During this waiting, window painting is momentarily suppressed.

That’s why the program runs faster: No window painting means less CPU spent updating something faster than you can read it anyway.