This method is evaluated during the startup hashing of all modules. This happens because part of the version hash, must be the message blob, and the message blob varies by two things: message keys, and language.
The preload logic for all 3000 modules on enwiki, currently spends about a 1/3rd of its time on this one module.
The list of message keys computed by this module class currently contains an expensive conditional based on local existence of a message (involving LCStore, MessageCache, and sometimes slave database reads).
If I understand correctly, the JavaScript code in this module also checks for existence via mw.message().exists(). As such, we could probably omit this and end up shipping the same data as otherwise, but with less computation cost.