title: Why copying memory from VRAM to RAM is slower than RAM to VRAM? (OpenGL)
body: <p>I am creating something similar to CUDA but I saw that copy memory from RAM to VRAM is very fast like copying from RAM to itself. But copy from VRAM to RAM is a way slower than RAM to VRAM.</p>
  
  <p>By the way I am using <code>glTexSubImage2D</code> to copy from RAM to VRAM and <code>glGetTexImage</code> to copy from VRAM to RAM.</p>
  
  <p>Why? Is there a way to improve it's performance like copying RAM to VRAM?</p>