On the maximum quality setting in the client, StreamingEnabled appears to incorrectly stream out terrain - that is, not streaming it out at all. This leads to a severe FPS drop when looking at terrain:
Compare this with lower graphics settings, e.g. half:
The place I am using to test this is Terrain gen 2.0 - Roblox which generates near-infinite terrain and uses StreamingEnabled so that the client does not have to have the entire map in memory at once.