Do you have a computer telling you that speed or are you just making a guess? Because I find it unrealistic to be +30 kph on flat ground with a mountain bike for 30 minutes.

I say this because I have a gravel bike and can only keep +30kph for long periods if I’m on a slight incline and I’m pedaling with a purpose (not full sprint, but you wouldn’t see a commuter pedal that hard)

On average people in commuting bikes will most likely be at around 15kph, low 20s on descents.