The MessageCache__get hook allows changing messages on the fly; it is invoked every time a new message is looked up, which happens several hundreds of times per request.
Typically a hook handler only cares about changing a small number of keys, but has to check every time a message is loaded whether it is one of those. It would make more sense to have a hook which is called at most once per request and allows handlers to register what keys they care about (and maybe map them to callbacks).
This would help with performance; it would also help with reducing false positives where MessageCache__get is called a lot so for requests that time out there will be a nontrivial chance that they happen to time out in this hook and create noise on unrelated dashboards.