Does showing a flashing time separator slow down a Mac? Is there any reason to be concerned that showing flashing time separators would slow down my Mac, since the computer now has to keep flashing the separator? What about showing seconds? Why or why not?
No.
The menubar is controlled by the process SystemUIServer. It requires little performance with &quot;flash the time separators&quot; enabled.
There are other processes which are more significant for the CPU performance.

This screenshot is taken with flashing time separators and show seconds enabled.
You can easily verify this by resetting the menubar using:
killall -KILL SystemUIServer