You may be interested in the twin paradox.
Imagine you're moving directly away from the Earth at sqrt(3)/2 c. From your point of reference, it's moving away from you. Someone on Earth has a set of mirrors with a photon bouncing between them. The mirrors are perpendicular to your path, so there's no length contraction. (That part can be proven separately, if you wish.) For simplicity, let's assume the mirrors are a meter apart.
From your point of reference, the x component of the velocity of the photon is sqrt(3)/2c. The total speed is c.
v_total^2 = v_x^2 + v_y^2
c^2 = [sqrt(3)/2 c]^2 + v_y^2
v_y^2 = c^2 - 3/4 c^2
v_y^2 = 1/4 c^2
v_y = 1/2 c
Since the y component of the velocity is half the speed of light, the clock will only run at half speed from your point of reference. It will run at full speed from the point of reference of someone standing right next to it, so from your point of reference, they're moving at half speed.
Separate thread. Wikipedia often does not do a great job on this type of topic for some reason. Example:
...The standard textbook approach treats the twin paradox as a straightforward application of special relativity. Here the Earth and the ship are not in a symmetrical relationship: the ship has a turnaround in which it undergoes non-inertial motion, while the Earth has no such turnaround. Since there is no symmetry, it is not paradoxical if one twin is younger than the other. Nevertheless it is still useful to show that special relativity is self-consist
It has been suggested that animals have less subjective experience than people. For example, it would be possible to have an animal that counts as half a human for the purposes of morality. This is an argument as to why that may be the case.
If you're moving away from Earth at 87% of the speed of light, time dilation would make it look like time on Earth is passing half as fast. From your point of reference, everyone will live twice as long. This obviously won't change the number of life years they live. You can't double the amount of good in the world just by moving at 87% the speed of light. It's possible that there's just a preferred point of reference, and everything is based on people's speed relative to that, but I doubt it.
No consider if their brains were slowed down a different way. Suppose you uploaded someone, and made the simulation run at half speed. Would they experience a life twice as long? This seems to be just slowing it down a different way. I doubt it would change the total amount experienced.
If that's true, it means that sentience isn't something you either have or don't have. There can be varying amounts of it. Also, someone whose brain has been slowed down would be less intelligent by most measures, so this is some evidence that subjective experience correlates with intelligence.
Edit: replaced "sentience" with the more accurate "subjective experience".