I pedal 300 W and I only reach 30 km/h? Something is wrong with my #roadbike. Do I need a new bike?

The bike is a Cube Attain SLX with a Shimano bottom bracket and DTSwiss E 1800 wheels. Is it my posture? Is the bike too heavy? What am I doing wrong? Normally 170 Watts should be enough for 30 km/h. My Powermeter is the Garmin RS 200, calibrated every 2nd ride.