Manual:$wgAdaptiveMessageCache

Message Cache: $wgAdaptiveMessageCache
Keep track which messages are requested and load only most used messages.
Introduced in version:1.18.0 (r71412)
Removed in version:still in use
Allowed values:(boolean)
Default value:false

Details edit

Instead of caching everything, keep track which messages are requested and load only most used messages. This only makes sense if there is lots of interface messages customised in the wiki (like hundreds in many languages).