Check out this thread, specifically the reply I linked to:
(Some good stuff below the reply too, like bug fixes)
Just a heads up, Roblox can’t do portals the same way that Unreal/Fortnite does (separate rendering stacks), so Roblox is basically inherently less efficient. In addition, the resolution of the output is limited by the aforementioned inefficiency, and Roblox has capped the resolution in advance.