The way I look at it is that speed is dependent on the strength of a subspace field distortion, measured in cochranes, and the mass you're trying to move. The distortion you can create depends on the mass and efficiency of your warp coils and the amount of energy you can pump into them. If your warp coil and plasma conduit engineers are crap, your largest coils are small and inefficient and can't handle a lot of energy; the resulting field distortion is small. On the other hand, if your engineers are good, they can build a single nacelle with big, efficient coils that can take a lot of energy; so, the nacelle creates a powerful distortion. Not all nacelles are created equal.
So, with a given field distortion, I'd imagine that the speed you can reach depends on the mass you're trying to move. Endurance depends on how much fuel you have. It's all arithmetic!