Here is the proof you ask for:

MySQL stored routines: the compile cache is on the connection thread, not in a global cache, so stored procedures are re-compiled on each connection thread, which is a waste of resources (even if such connection-caches reduce global lock contention).

AFAIU, this design was chosen because it was considered too much of an overhaul to actually re-architect the pieces of the server kernel/runtime which would be necessary to implement a global compile cache, and thus we have a “half-baked” feature that although it works, isn’t efficient and causes perhaps more problems than it solves.

-jay