Is there a keyboard shortcut to pause the output of a CMD window while it's running?

Does your keyboard contain the Pause/Break key? It would serve well for this. Here's what it looks like, or at least used to:

As an interesting bonus, this key can also be used during POST (Power On Self Test, which runs when the computer is turned on) to read output from the BIOS.

If you don't have that key, the combination Control + NumLock should work just the same, and there are other options depending on your manufacturer, according to Wikipedia.

To resume execution, press Enter.

I didn't see this in the other answers.

I believe the serial control sequences of 'Ctrl-S' XOFF and 'Ctrl-Q' XON are the key sequences intended to solve the issue.

Please see;

XON/XOFF

You can use "more" command. Though it is not exactly what you are looking for, it will limit screen output by a page.