You only see significant speed changes if you change type of bike (i.e. MTB to road bike). an OK and fitting bike within the category will be very similar. You see some small advantage if you throw a lot of money at the problem. For example if you upgrade from old steel roadbike with cheap tires to all new carbon frame and wheel bike, but it will be a small increase in speed. the fact that the overall geoemtry between two road bikes is different, and your body creates different wind resistance on each bike probably contributes more to the differences. Geometry hurts (or helps twice: your body provides more power with better fit, and your body will be in the wind in a different shape/angle. So depending on your shape and fit, the old steel bike could actually be faster if the fancy carbon bike puts you in a weird position.

And yes, the motor determines speed. Many of the road bikes for just a fe thousand $ you can buy would be good enough for a TdF podium spot. You just need to find a better rider than yourself.