You pretty much nailed it, as profound as it might be, that’s exactly how they have accomplished those ViewportFrames. Now it’s all about how you come about making the code more efficient and performant on the local side of things. i.e RenderStep, etc.
Now where is it causing lag? Just the viewports? or the game entirely? Try showing less viewports? Just try to come at this with a problem and cause attitude. Find the problem, find the causes, and then formulate a solution… then repeat.
Just keep experimenting! Cheers.