Do you actually think CPU usage is a concern here? Database performance and optimization, horizontal scaling, cache optimization (and cache invalidation) are the big problems. Security is also a big problem.
The performance critical part is the database, and that at least used to be written in Java (cassandra db).
Writing the bit of glue code between your high performance database and your fronted in cpp would introduce all kinds of really bad potential bugs, scary remote code execution bugs.
CPU usage of that glue code might take up 10% of your hardware budget worst case, even with a "slow" language. It's just not a concern.
We're not talking about 10% factors, but 10x factors between slow and fast languages. People say CPU usage doesn't matter when you're horizontally scaling, but horizontal scaling increases your ongoing operational costs, and you often are horizontally scaling precisely because your code is too slow.
The performance critical part is the database, and that at least used to be written in Java (cassandra db).
Writing the bit of glue code between your high performance database and your fronted in cpp would introduce all kinds of really bad potential bugs, scary remote code execution bugs.
CPU usage of that glue code might take up 10% of your hardware budget worst case, even with a "slow" language. It's just not a concern.