As the drivetrain started to wear, I noticed that I constantly have to tighten the barrel adjustor on either the shifter or rear derailleur to get the bike to downshift properly. Otherwise it will lag a few seconds or not shift at all. Its very difficult for me to get the line tension dialed in for both up/downshifts for very long. I'll see if replacing the chain will help with it.
Also, my e bike downshifts rather roughly under power, even though it's designed to detect gear shifts and cut power. This will definitely put extra wear on the rear cassette as well as chain.