I don't think so: the human mind doesn't run on a Von Neumann architecture, so any speed/search depth tradeoffs wouldn't behave in a computer-like way and a lot of other intuitions about computational performance don't apply. All else being equal, uniformly slowing down the human brain's operations probably wouldn't impair its ability to exhibit behaviors we'd recognize as conscious; they'd merely increase the time it takes to get an output given some input. Most importantly, human-unique capabilities like language would probably be left more or less intact (if slowed down).
A lot of animals actually have significantly faster nerve conduction than humans, although I don't know if this extends to the brain.
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".