使用SDL2纹理渲染每像素的慢像素

I am trying to create a image pixel-per-pixel and display it on the screen with SDL. The image has to be refreshed and displayed again ~50 times per second (I am looking for a 50 FPS game). I tried to create a simple program to illustrate what I want to do: I create a 1280 * 720 window and texture which I alternately fill with green and red. The problem is that the code runs very slowly (~8 FPS). Where did I mess up?I am trying to create a image pixel-per-pixel a