[Aeolun]

This isn’t super surprising. Going 5km/h faster when you’re already going 20km/h takes just as much effort as going those 20km/h in the first place.

If you keep pushing yourself to 25km/h at the start, you’ll end your ride exhausted and going 15km/h.

[ajuc]

Also depends on topography and road quality and where you choose to push hard vs relax. Same effort on a bad part of the road will gain you less time and waste more energy than if you relaxed on that part and pushed hard on the good parts.

I've noticed recently that I'm more exhausted (to the point of "hitting the wall" and doing under 10 km/h for the last 5 km) when I'm going 30 km to my parents than after 110 km ride from Lublin to Kazimierz Dolny and back. The main difference is that the first half of the road to my parents is on a shitty brick-paved bike path and it's consistently slightly uphill for like 20 km. The other road is mostly asphalt country roads and small hills and downs every 100 meters so you can rest every few minutes.