I wanted to document this because I spent a long time tracking down a severe Mass AI performance issue, and the eventual fix was not obvious.
Engine: Unreal Engine 5.6.1
Systems involved: Mass Entity, Mass Crowd, ZoneGraph, StateTree
One potentially important detail: this project had recently been upgraded from UE 5.6.1 to UE 5.8.1, tested there, and then reverted/backported to UE 5.6.1.
I cannot prove that the engine-version round trip caused the problem, but it is suspicious enough that I think it should be mentioned. After returning to 5.6.1, the existing ZoneGraph Data appeared valid enough to build and run, but Mass agents later showed invalid lane-location behavior that was only fixed by deleting and recreating the ZoneGraph Data entirely.
The Problem: So I had about 2000 mass units moving with very stable fps…above 60. Then after 5.8.1 try then revert. The FPS went down to 6 with 1000 units. I was then being spammed with [MassZoneGraphFindWanderTarget] Invalid lane location.
If anyone else hits this after moving a project between Unreal versions—especially 5.6.x ↔ 5.8.x—I would strongly suspect stale or incompatible generated ZoneGraph data.
