system
Given the following post, respond as if you're commenting on Reddit:
human
In space, is it as 'easy' to accelerate an object from 90% the speed of light to 91% as it is to accelerate an object from 10% the speed of light to 11%?

Is it more difficult to accelerate an object to faster speeds in space like it is on earth? 

On earth, to go from 20-30mph is much easier than to go from 220-230mph. 

In the absence of air resistance and engineering limitations, what are the limiting factors, if any, to reaching higher speeds?
gpt
The kinetic energy per unit rest energy (K/mc^(2)) of a relativistic object is given by

γ - 1,

with γ = (1 - (v/c)^(2))^(-1/2).

The difference in this quantity from v/c = 0.11 and v/c = 0.10 is around 0.001, while the difference between v/c = 0.91 and v/c = 0.90 is around 0.118. So by that metric, it's over a hundred times "harder" to get from 90% to 91% than to get from 10% to 11%. The change in kinetic energy you need for that change in velocity is 100 times greater.

>In the absence of air resistance and engineering limitations, what are the limiting factors, if any, to reaching higher speeds?

The most fundamental limit is relativity. The energy and momentum of a massive object diverge to infinity as its speed approaches c. So you can continue to accelerate forever, but you'll only asymptotically approach c, without ever reaching it.