No, it wasn't, but its efficiency should not be horrible, like 50% less.
Could be anything, from battery issues to cooked magnets. Pretty hard to cook an old 400 motor, but possible if it was ever fed 72v for sure.
I'd be suspecting batteries that are weak, perhaps not able to stand the discharge rate, but another possibility is the 400 motor is a poor winding match with your wheel size, and or the type of riding.
Efficiency in some types of ride can vary greatly. For example, the 406 is a bit faster wind motor and it is asked to start and stop a lot, it will not be running near as efficient as a slower wind motor like a 408, and way less efficient than a slightly slower wind geared motor, when asked to stop at a light every block. Both run about the same at speed, but one starts up more efficient. I find a 20% difference is common, but not 50%. 20% was comparing a 407 type motor with a larger diameter 7 turn dd motor, or comparing the large 7 t dd with a 10t geared motor, with a ton of stop signs on the route.
But on an open flat road, both would run with near identical efficiency.
And then, there is how hard you pedal.
Basically, you are running an experiment with 3 or more variables, which is not science. only one variable gets you science. You have two riders, different motors, and different batteries. Id expect some noticeable difference.
Repeat the test with you riding the same route, same pedaling effort, and same weather, using the same battery on both bikes.