The extra weight of an ebike makes very little difference to rolling resistance on the flat.
It apparently does to the OP 😉
Forget trying to pedal above it as the bike is that heavy/laggy you’ll expend more energy than a normal bike.
Is probably a combination of the drag above the motors assistant, and the extra weight. I bet on a light 12-13kg ebike it would be much easier to keep up to speed.
Tbh, it sounds like the OP is annoyed that his fancy new ebike hasn’t magically made his commute easy.
As for speed, yeah most ‘cyclists’ won’t do much over 15mph on the flat, but anyone with a bit of fitness should be doing more. I’ve just checked, and on my 9.5kg gravel bike with 47mm tyres (and carrying a 4-5kg rucksack!) I’m sitting at anywhere between 20-30mph on a flat section of my commute. If there’s a nice 5-10mph tailwind, sitting 25-30mph doesn’t take that much effort.