Unity 3D - Render texture from camera low resolution and quality

I created a render texture and attached it to my camera. I put the texture on an object, and the quality is really low, also the resolution. In scene mode, it looks good, but in-game mode, it looks bad. So I tried to increase the size of the texture. It helped but it really slows down my game. A lot of stacks and low FPS, when I set it to the best size.

How can I set the texture to show the best resolution and quality without slow my game?

This how it looks bad in-game mode:

[176427-צילום-מסך-60.png|176427]

Change the size of the render texture to something like 500x500