Currently, as far as I can tell, Microsoft graph offers 2 primary endpoints for outlook mail folders according to https://developer.microsoft.com/en-us/graph/docs/api-reference/v1.0/resources/message
List mail folders and List child folders - meaning just to build the folder hierarchy in my dashboard app, I need to make recursive REST API calls - which is very slow and very bad.
Is there any way to get microsoft to just return all folders at once?
No there isn't. You should check the childFolderCount property on each folder and only query if non-zero.