One thing I discovered recently that could have been contributing, is that my rear brake seemed to be slightly rubbing my rotor. I was riding on a longer ride a few weeks ago, and it got worse. So much so that I actually removed my rear brake pads to continue my ride.My new SL is coming this week, but I get passed by roadies on my 500W, 65 Nm hub-drive all the time on flats and descents. So no expectation that the SL with half the power and torque will be any different.
Understand your situation and goals are very different from mine. I've made my peace with it at 76:
1. Usually ride alone and am seldom in a hurry.
2. Prefer to ride at lowest assist whenever possible, and my level 1 of 9 is pretty low.
3. Keep repeating to myself that my Tour de France days are over, and that there will always be those lesser and greater than myself.
Also keep getting passed by school kids idly ghost-pedaling RadRovers. That's the part that kinda hurts.
When I got home, I discovered that one of my pistons on my rear brake was out really far and the other was pushed in all the way. I could push the one that was out, back in using a tire tool, but it would keep popping out. The "pushed in" piston was stuck, causing the other piston to extend too far. I pushed in the extended piston and held it while pump the brakes a bit. This caused the stuck piston to push out. Pushed them both back in, inserted pads and seems to be working fine now.
Anyway... Moral of the story.. Could also be some equipment failure mixed with being old
Check your brakes!