Are you sure that the parking brake (if it has one; my ICE has a rim brake on the rear wheel) was disengaged? It's pretty easy to forget about it when your mind is on other things. I know someone who rode with the brake almost fully engaged
the other day, for just over 10 miles. Thought he had a bad headwind, or something wrong with his legs.
I guess you checked that the wheels spin freely when you lift one at a time off the ground? The 26" wheel will give you a slightly higher gear range than the same sprockets on a 20" wheel. And the extra weight of the HP with the suspension and giant battery is not to be sneezed at. Without counting teeth on your cassette and chain ring and diving into Sheldon Brown's gear calculator, it's hard to say what the difference in the drive train may be.
Check the chain path and the chain tubes; make sure you don't have a chain tube binding the chain against a pulley, or vice versa. The chain could have been installed with a 180 degree twist. Rotate the chain in each direction where it comes out of a tube. It should have the same amount of play in each direction. If it resists rotating in one direction but goes easily the other way, it could have been twisted when installed.
If the front wheels were not aligned properly, that would also add rolling resistance. Good luck.
Edit: I should have said "twist" instead of "rotate" the chain. I did not mean for you to rotate the crank.
Last edited by DeadGrandpa; 04-16-23 at 06:07 AM.