How to prevent obvious/visual loading of the lighting and textures in packaged game?

I noticed that in a lot of games made with Unreal, for a brief moment, you can see the lighting and the textures being built/loaded whenever a new level is loaded. (textures gradually become sharper, shadows suddenly appear, etc)

How do I prevent this?

Get better hardware.
It’s because Texture Streaming is turned on. You can disable it in the Rendering Settings, but will increase loading times.

I have:
Intel i5-3470
Nvidia GTX 1070 8 GB

It still happens.

Or you can block the camera until things are loaded.

Does this make the screen black or just hang at the last level?

Set up a loading Screen ( There are many tutorials like this Creating Loading Screens - #50 Unreal Engine 4 Beginner Tutorial Series - YouTube). And after the level has loaded set a delay for The Widget. After that you have to work out how long it takes until the lighting is ready. If you have the time set the delay in the BPs before you destroy the Loading UI. Voila the loading process is hidden.

Increase the size of texture memory pool.
Config stuff…