How to detect when the world is done loading / streaming?

I’m experiencing considerable lag during the first moments of launching the game and I want to use a loading bar to hide the world until it is completely loaded and running at the intended rate.

I’m using World Partition, and I have tried using Is All Streaming Completed from World Partition Subsystem, but this returns True before the game run smoothly. So I’m wondering what other methods exists for detecting the current state of world?