Trunk rack and backpack probably a tie.

Pannier will be anti-aero compared to a backpack, so probably slower @ same rider power output measured @ cranks.

Some backpacks will make you slower. Some sloppy backpacks you can't even stand and pedal with, good ones with sternum straps and good load handling will let you go anywhere.

Need to hit a couple of shops enroute then backpack wins more time since you don't have to remove panniers or trunk bag from rack upon arrival at each store, then remount them upon departures.