Use a color buffer of intermediate modification step (and not the draw pixel FBO's texture)

[ClementineAccount]

Don't call glTextureSubImage2D() for each pixel drawing. Instead, call it only at the end of every frame that has modification once the intermediate color buffer similar to what I did in GAM150's pixel grid.

After that, may consider using a PBO to upload the pixel data directly #1 to the texture (or a compute shader #2)

[ClementineAccount]

Done