Hey there, this sounds like a bug. Waze usually displays speeds *under* what your current speed readout is on your speedometer (if your GPS is good, which it should be, your Waze speed is more accurate than your car's speedometer) This is due to safety/legal reasons on car manufacturers' behalves.

You can head to Waze's Help Center and contact support to see whether you're experiencing a bug or whether that's intended behavior.

I can't post links here, but just google "waze help center" and you'll find it.

Do note that if you're facing some sort of bug or unexpected behavior, they will likely ask you for screenshots/screen recordings and debug logs so they can investigate further.

You can be proactive and submit a ticket to them with all the information at the ready.
Just search their help center for "submit debug logs" and you'll find the steps.

You basically just enter debug mode, restart Waze, *reproduce your issue*, then hit the report button (the orange one on the map) and click the Debug (wazer bee) icon and wait for the confirmation message.
At that point, you can contact Waze customer support and they'll be able to investigate your issue in a manner that suits all parties.