Render last scene screenshot when scene is not being modified

The 3D scene of my iOS Unity game is static (not being modified) about 50% of time. You can move and rotate the game objects inside the scene, but sometimes you interact only with the UI system which is rendered on the top of the 3D scene. When you interact with the UI system, 3D scene is not being modified (so it is static).

The 3D rendering is very heavy and makes the UI system sometimes very laggy (or choppy). My idea of optimization was to capture a 1:1 screenshot of the 3D scene and render this screenshot instead of the 3D scene (when user does not interact with 3D scene). Hypothetically this should drastically increase the performance when user interacts only with the UI system.

Does it take a lot of CPU time to take a 1:1 screenshot of the Unity 3D scene camera?

Would it be fast enough for switching between a rendering-screenshot camera and a rendering 3D scene camera?

Thank you!