I would like to report that I am also having issues with Guard Spawners.
I am aware that AI agents generally have a good bit of performance overhead, but I have wildlife scattered around the creative map that I am currently working on with no issues. However, I am getting large frame rate drops when being in an area with about 4 Guard Spawers. The discrepancy leads me to lend support to this Issue Report. Something about the Guard Spawners is causing a laggy bottleneck. Particularly when using more than 1 in a single area.
4 Wildlife Spawners within 100m: Almost no frame drop. Fortnite handles it like a champ.
5 Guard Spawners within 100m: Framerate TANKS when entering the area, but slowly recovers over about 40 seconds after entering the area.
Not sure what is up here, but definitely worth the UEFN/Creative team looking into. The Guard Spawners are doing something very inefficiently compared to the Wildlife Spawners.