Originally Posted by
telkanuru
The more the tyre wears, the flatter it becomes and the more surface area is in contact with the road, and so there is more rolling resistance.
That's a thought, but the size of the contact patch doesn't play as much into rolling resistance as people think. (The tread moves up and down off of the road, so it's not scrubbing against the ground unless you're skidding it.) Since the tires are getting smoother, the OP could afford to put a couple more psi into each tire for less RR due to sidewall deformation.
That being said, there's no such thing as sustained athletic peak performance. Any decent competitive athlete understands that you can't maintain peak conditioning forever, and athletic training for everything from running to fencing is designed to ramp up to a competition. I'd say the OP needs to take it a bit easier every once in a while, and that such fatigue is part of the natural cadence of exercise.
I bet this is playing a part -- might be time to take it easy for a bit and recharge.