I could be wrong in my thinking so someone correct me if I'm wrong.

Higher load springs could make it tougher.
The cam profile as well, since it's changing when valves open so more pressure may be building?

Almost sounds like the battery itself could be dieing. Ok voltage but low amperage. I hate trying to diagnose over the internet. Sucks ur all the way in canada. I'd drive an hour or two, but thats too far lol.