Made this a while ago, decided to clean it up and post it especially since its been mostly made redundant as of a year ago.
Works by creating ISM or HISM components within the bounds of a volume.
It can discriminate between differing Static Meshes and Materials and uses a custom made Axis-Aligned Bounding Box formula to detect meshes without collision.
Capable of quickly duplicating & updating instances.
Note that the file name can be saved or manually set to allow loading across different levels.
Supports a working grid mode which was designed for large scale, quick and dirty instancing of an outdoor environment. Above example is 35K meshes being stored between a 110 Volumes.
This system is blueprint-driven for maximum compatibility for version upgrades.
Depending on whether the meshes are static, with baked lighting this tool would improve the frame rate between 35-60%. Post 4.22 it will still get numbers around 5-30% but only if the actor instances are using movable or stationary lighting.