He says in the opinion:
Some of these arguments might persuade someone that the death penalty should be abolished, but do nothing for Breyer's legal argument. Eg, a judge could compensate for lousy lawyering better than a jury. Other nations don't even have a right to a jury trial. The prolonged wait is caused by judges like Breyer who don't believe in the death penalty anyway. And the last statistic is almost completely meaningless. It might also be true that 3% of the counties have 50% of the nation's murders. Much of US crime is concentrate in a few urban counties. It is a textbook example of lying with statistics because it is intended to show that the death penalty is applied inequitably in the US, but it actually doesn't show that at all.
Volokh's warblog has some additional statistics related to this point.