In the options set
- Portal Resolution Scale = 100
- Portal Render Quality = same as main render
- Sum Portal World Detail = 200% ( or 100% * Maximum Open Portals)
The client still has to create the meshes for the world so that will take some seconds but once its there it should be the same.
In places like towns there can be dozens of portals so your cant load all of them, its too many connections for the servers even if your PC could handle dozens of worlds (each world can be 1gb on the ultimate settings plus the cpu load per world/area).