Temporary render texture resolution affects performance or not (Graphics.Blit)

Hi. When we use smaller temporary render textures than the source, it can gain performance or not?
How does Graphics.Blit work when source and temporary render textures do not have the same resolution? interpolation or etc.
And do we need to write GL.Clear like below?

Code (CSharp):

1.  tempTexture = RenderTexture.GetTemporary(currentWidth, currentHeight, 0, RenderTextureFormat.Default);
2.             Graphics.SetRenderTarget(tempTexture);
3.            // GL.Clear(true, true, Color.black);
4.             Graphics.Blit(src, tempTexture, Materials[0]);
5.             Graphics.SetRenderTarget(src);
6.             //GL.Clear(true, true, Color.black);