Explicit use of vector intrinsics are not the source of any 10x performance boost, nor anything to do with C++ in particular. Again, only a small (but probably not minuscule) portion of the difference has to do with Java vs. C++. The bulk of the difference is due to all sorts of optimizations, most of them could have been done in Java as well. But the ScyllaDB people are more experienced in C++ than in Java, and as they use sequential code anyway, there isn't a big downside for using C++ -- certainly not for them -- so it was the better choice. From what little I know, the reasons why such optimizations weren't done in Cassandra are because 1. the people working on it aren't low-level optimization experts, but more importantly, 2. because the performance was good enough.