Kafka is focused more on throughput rather than latency: if latency is critical, you should use a database rather than a message queue (may be with some caveats, e.g., for finance, telcos, or air traffic control; TIBCO et al cover that market well, however).

Kafka's use of the JVM does not really impact throughput. Very little portion of the messages' lifecycle is spent in the JVM heap, Kafka makes aggressive us of the OS page-cache and avoids copies (e.g., using sendfile() whenever possible).

I agree that, e.g., if Kafka had to make heavy use of in-process memory (this is appropriate for databases) as opposed to OS managed buffers, then a language without a garbage collector (or perhaps a garbage collected language that gave you an option not to generate garbage in the first place...) would help.

That said, there's other advantages to using C++, but they won't bring significant performance improvements -- although I suppose one could experiment with, e.g., using AIO but that might end up causing more harm than good in case of Kafka.

As the "slow compilation times" feature, scalac and sbt do a much better job of it than cmake and g++/clang :-)