I ride in a very hilly area, typically 100ft climbing per mile. My "normal" average speed on a reasonably long (say more than 30 mile) ride is about 13.5mph. On a very flat ride in Cheshire, for example, I will average about 18mph
The reason is that the wind resistance is proportionate to the square of the velocity. If you do the maths, maintaining a constant speed results in less wind resistance than some fast riding (downhills) and some slow riding (uphills). That is the reason that hilly riding is slower than flat riding
The effect is marked. In the Peak District even the local professionals struggle to do more than 18mph on a hilly ride. The other speed killer is when the downhills are small, narrow roads where you have to scrub off speed to remain safe. I hate that