In my experience, microservices/backends are almost always I/O bound, and the CPU usage is almost pure overhead (meaning it may exist, but you could reduce it with effort - it’s not intrinsic to the problem), unless the service is doing some particularly domain-specific compute-intensive task.

For your typical website backend between frontend and db, you are doing some async conversion of JSON to db call back to JSON. For an HTTP microservice you are also typically converting some JSON request body to a JSON response body with some kind of I/O call in between.

So that’s a roundabout way of saying I think that the case where majority of CPU is spent on SerDe is more common than you think. And it’s not necessarily a problem if the effort to improve is not worth the savings.