Manual:$wgForceDeferredUpdatesPreSend/en

Details
Force deferred updates to be run before sending a response to the client, instead of attempting to run them after sending the response. Setting this to is useful for end-to-end testing, to ensure that the effects of a request are visible to any subsequent requests, even if they are made immediately after the first one. Note however that this does not ensure that database replication is complete, nor does it execute any jobs enqueued for later.

There should be no reason to set this in a normal production environment.