Does showing a flashing time separator slow down a Mac?
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