Even in a database, most of the code isn't performance sensitive. Making your life a bit harder in the fast path so it's easier in the slow path is at least a tradeoff worth considering.
So if you're latency sensitive then all of your code needs to be aggressive at avoiding object creation. All of your code becomes part of "the fast path", even if it's in a different thread.
Or you isolate your fast path in a different process or a non-GC'd runtime, the later being the approach taken here by Instagram.
[1] http://15721.courses.cs.cmu.edu/spring2018/papers/02-inmemor...