The correct answer is that Einstein was wrong and Tesla was correct, that gravity is the fastest direct propagation speed in the universe. But there is also a speed of interference propagation that is almost instant across the universe.

But, you probably want to understand the physics as it is taught today in physics books. This is where relativity comes into play.

The two space ships, each travelling at 0.8c away from the planet. But, to each other, they are travelling about 0.96c away from each other. What is happening is that space is getting contracted along the axis of the two ships.

Not only does time get slower, but also space shrinks.