Just a guess, but I'm thinking you were watching a rookie rider.
I know you aren't a big hub fan, but to get it right, you have to come up with the load/effort (wattage) you're willing to contribute personally, your desired pedaling speed/cadence you want to do that at, and match that with your desired ground speed and the rolling resistance of the surface you are on, along with wind and hill conditions. Bottom line, there's a LOT more to it than just jump on and go. Most new owner/riders soon see there's much more to it than meets the eye.
The guy you were watching may have just had the bike in the wrong gear for instance....
As far as battery consumption, throttle vs. PAS, they should be close to the same - whatever is required to move the bike with all of the conditions mentioned above. Big factors will be what you are contributing personally, and how fast you're going.