posterizeTime() lets you simulate a lower fps than that of the comp, by specifying a number between the paracentesis.
Make sure you don't place it at the bottom of your code, as it does not return any value.
Posterize the expression result
Write any additional code underneath your posterizeTime() statement, like so:
Because posterizeTime() expects a number, we can even use the pickwhip tool to use a slider value as the new fps.
Posterize time the original value