Which is why we should whack Mars with monster spacebergs (at full speed) before it gets populated. To heck with slowing them down.
I wasn't planning to slow them down. Not sure where that came from. But as I said, I see no economic or technological advantage to one block of ice of 30,000 tons versus 30,000 chunks of 1 ton each. The latter would be inherently safe so I would go with that.
If these things were being produced with human labor or with machines made by people, there would be a big advantage to big chunks. I get that. Fewer launches. 30,000 times less work. Perhaps you are looking at it from the point of view of the 21st century in which people still do manual labor. Think about how it would be when the robots, all of the work, all materials and machine tools are products of self-replicating technology. Why would anyone care if the robots do 30,000 times more steps than they would otherwise?
Suppose you run a program once a day. It is poorly written (as many programs are today), so it takes an extra 10 seconds to run. You are polishing your glasses or stirring your coffee, so you don't even notice. The computer has done 30 billion operations more than it needed to. Well, so what? Do you care? In the 22nd century, if a bunch of robots in the Oort cloud -- robots that no human has ever set eyes on, or ever will see -- do 30,000 times more work than they would have otherwise, why will anyone care? As long as they get it done. They will cost virtually nothing, just as the extra 30 billion operations cost you nothing.