2.5km is nothing.
Streaming was still on. Just not on the landscape.
They still had all the impostors and HLODs they needed.
Up to 4km square can do fine even when using landscape and even on a 1080 powered potato.
Provided you follow best practice ofc.
Using meshes only and a good replacement system you can easily do without any streaming up to 20km^2 the problem becomes the instance counts. You start needing a beefier non-potato CPU to get things to run and replace nicely.