Additive Load Scene Async is too demanding; how do I fix?

I'm working on getting my game to stream in all my content seamlessly.
I'm testing some triggers to call SceneManager.LoadSceneAsync to add my next scene into the game, but the process drops way too many frames; the game visibly hangs while the scene finishes loading.

First off, I'd like to know if anyone knows how I can fix this problem, or can suggest something I can look into.

I did find https://docs.unity3d.com/ScriptReference/Application-backgroundLoadingPriority.html which...

Additive Load Scene Async is too demanding; how do I fix?