Level Streaming and How to make it the most cost effective solution

I am trying to make a level in my game where when I press a certain button, the level goes into another “dimension” I already made the other dimension levels, but with level streaming the process takes way too long, are there any other ways I could do it instantaneously?

Have both/all of those levels opened up at the same time. For example have the dimension B located 10000 units below the dimension A.
Instead of making the another dimension appear to your character, with the button press make your character teleport to the position of the another dimension.
If you want your character stay at the same spot between dimension warps, have his location in dimension A saved and add that to the offset of the another dimension. Thus, make the teleport position current position -10000 for z.
This is how “battle screens” in RPGs are made for example.

This method obviously takes 2x the ammount of memory, but if you want the transports to be instaneous, this is pretty much the way to go, since you cant start loading data into memory at that point anymore. Save the memory elsewhere, if that starts to become a problem.