Performant Alternative to render texture?

I am trying to create something similar to a periscope, but I do not want to use a render texture, I am worried this may greatly decrease performance at higher resolutions because I will have more than one active render textures at a time.

Is there another solution to using Render Texture? e.g. an asset

Thank you!

if you set those render textures to less resolution(256X256) it wont give you any problem