Bicycle is harder to pedal when standing
Something is dragging as you pedal harder. The most likely thing is that your rear wheel is moving so that the tire drags against the chain stay when you apply power.

This is something that you'll probably need to troubleshoot when you're riding because it isn't likely to show up with the bike on a work stand. You might be able to make it happen by applying a standing load to the pedals while holding the front brake. To do this I'd want somebody holding the bars to stabilize the bike (and to help me look for what moves when I load the chain).

The interesting thing about what's happening is that you say the problem goes away when you ease up. If the problem is due to the tire rubbing on the chain stay then there must be some mechanism to return the wheel to its neutral location. The only thing that I can think of right now that might do this is a nutted rear axle where the nut on the right (drive) side is loose enough to allow the axle to move under load and then move back when the load eases up.