I would surmise that the problem is the map was scaled. Which also deteriorates performance and may not be needed.
Always use instanced static meshes whenever possible.
This engine is horrible at rendering as of the past 2 releases.
We are talking 1000 pysics simulating cubes being a halting drag with only one directional light.
That means that foliage around the map will eat performance like there’s no tomorrow too. If casting shadows.
Overall, you can make infinite levels just fine with the automated world composition system.
Origin rebasing just works. You can load levels in manually as you progress.
Change level positioning at runtime too with some work.
However, working hands on on a 200km x 200km map is demanding on all sides, particularly hardware.
I would suggest starting off with the largest landscape component size. 8km^2 per tile…
There’s 2 versions that allow for that. They perform differently so you should run a test with both versions to baseline your fps/ms counts.
Manually load in 4 levels, sit at a point where you visually see all of them after FreezeRendering and you’ll know if you can actually afford to use unreal for your target system/resolution.