Memory Performance - Registered vs Unregistered

I've been trying to do some research into this but haven't been able to find a straight answer.

I know that Registered memory is more stable and consumes less heat/power.
The performance difference is what I'm interested in knowing.

Articles explaining the difference say that Registered runs at "one less clock cycle" than Unregistered. But what does this mean, exactly? 1 mhz lower, or something else? Is there a noticeable performance difference or is it generally imperceptible?