[CJefferson]

I used to often hear people joke that because of Moore's law, there was no point running any computation which would take longer than 4 years, as you'd instead be better to wait 2 years for computers to double in power, then run your problem then.

Interstellar travel probably has a similar problem, even if you have an engine that can do 0.1c, you have to be sure in 5 years you won't have one that can do 0.2c, or that ship would beat you.

[nradov]

Propulsion technology advances much more slowly than Moore's Law. The rate of progress in terms of ISP or maximum velocity has been approximately zero since 1969. The only way we'll ever be able to afford sending a probe to another star is with some new breakthrough technology, and disruptive technologies don't appear on any predictable schedule.

[tuatoru]

In addition to nradov's point: double your speed, quadruple the blast energy of collisions along the way. Interstellar space (in galaxies) is not as empty as we could wish.

And the relativistic mass equation imposes a very lower upper bound. Light is slow.