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?
The menubar is controlled by the process
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:
I don't think it will do any harm. You Mac is using seconds anyway, only now he will actually display them. And about the flashing seperator, I would be too concerned about that either.
Basically, showing something on the display is a matter of turning pixels on and of. So your Mac has only work a tiny bit harder to turn those pixels on and off again. Maybe, their will be a very very very slight less battery performance, but nothing that will really slow down your Mac I guess.