From looking into this locally, it seems this is caused by very high server frame times caused by the nav mesh being constantly re-dirtied. This is caused by a few very large static mesh actors in the scene that are constantly moving. We should feed back to the creator that they should disable the “CanEverAffectNavigation” option on their large moving static meshes, eg:
The main large moving static mesh actors seemed to be all the P_Sloops, the P_Windmill & P_WindmillBlades, and P_SkysphereSun.
