# Less speed increase obtained per amount of energy as approaching speed of light?

I found myself in a dichotomy when I tried to explain someone why we start getting diminishing returns on speed vs the energy investment as the speed of light is approached.

Is it because of relativistic mass (which would indicate that the object becomes harder to accelerate)?

Or is it because of the way relative speeds work in the fabric of the universe, which, considering the Lorentz factor, makes it mathematically impossible to produce relative speeds higher than c?