I have seen terrible C++ code and it takes long to get correct. The arrogance of "C++ code is fast" is well known.
With Golang they could have written the whole thing faster with similar speed. But even then, I am also suspicious of the C++ is 10x faster than Java for this kind of task.
The frameworks or the quality of java code make a huge difference. I am even tempted to take a look in the Cassandra codebase to see what went wrong. Maybe the problem is that they have not updated to Java 7 or 8 practices which in my opinion is half of performance lost.
The other half is that people try to have something working correctly and optimise later.
But even in that case, I could still try C or D before delving into C++.
I really don't buy the whole argument.