You are placing way too much weight in microbenchmarks. You simply can't use them to make a sweeping statement like you just did. Writing code that is identical to one another from language to language is not idiomatic and is not representative of how you would write each in a large scale project such as cassandra.

Java has a ton of overhead that C++ doesn't. Each object has metadata which results in more "cold data" in the cache. Each object is a heap allocation (unless you're lucky enough to hit the escape analysis optimization), which again leads to less cache locality because things are distributed around memory. Then there's the garbage collector. Then bounds checking.