World Machine will soak up tons of RAM if you let it. If you don’t, then it will spool stuff to HD. But then building a 50kmx50km map in World Machine under those conditions, and depending on the complexity of the graph, may take several days. I built a single 16k resolution map that started from a GeoTiff, and that alone took my rather beefy machine about 8 hours to render.
But once you chunk all that up into tiles, each of which could be 2k-ish in size, importing them into UE is no big deal. In my case it took about 2 minutes to import all of them (64 2k height maps). Using World Composition, you don’t have to have all the maps loaded at once. When I’m doing detail work, I only have 4 or 6 maps loaded at once.