Your tutorials don't read from video memory - that's why they run at a reasonable speed.
@man id unknown,
You can use the SET CAMERA TO IMAGE command to redirect your rendering to an image, then display the image using a sprite. Try to render to one image, convert to a memblock, manipulate the memblock, then convert back to a different image - show that final image as a sprite, or paste it to the display.
If you absolutely have to read from the backbuffer, then don't do it a pixel at a time - read the whole thing into a memblock, manipulate the memblock, then write it all back at once. It's better to have one long read than many small ones.