Rockets famously take exponential amounts of fuel to reach higher speeds. I'm a layman, but my guess is that this comes from the exhaust speed being fixed. Orbital speed is higher than exhaust speed, so from a frame at rest the rocket leaves behind a bunch of propellant moving in the same direction as it went. That's wasted energy.

Back-of-napkin calculation says that if you managed to perfectly match exhaust speed with current speed, leaving all the expelled propellant stationary, it would only take quadratic amounts of fuels to reach higher speeds. Like the kinetic energy equation predicts.