Here is a link to a course by Sjoerd de Jong on what is going on under the hood for the rendering engine:
An In-Depth Look at Real-Time Rendering - An In-Depth look at Real-Time Rendering (epicgames.com)
It was my understanding that static meshes are automatically instanced to reduce draw calls, whereas Blueprints are not.
When I tested it using, there was noticeable performance differences between instancing static meshes vs blueprint “containers”. Whereas the blueprint versions could not be instanced.
This may have changed since 5.0 - 5.2