This reports the port's operating speed: the configured value when the port's clock source is internal, or the speed detected from the incoming signal when the clock source is external; a value of 0 with an external clock source specifically means no clocking signal is being received from the attached device at all. This distinction matters because a 0 reading doesn't mean 'slow,' it means the far-end device isn't providing timing at all, a very different fault to chase down. For example, a technician seeing PortSpeed at 0 on an externally-clocked port would check the physical cable and attached DCE equipment rather than suspect a configuration mismatch.