Assuming he's talking about Factorio:
You have this game world, split into large chunks:
You press the map button and it looks like this:
Note the square edges. Those are chunks.
But without seeing the rendering pipeline, I agree with the two answers provided. It's most likely an issue copying to system RAM. You should definitely try to avoid system bitmaps for live updates at all cost. Shaders are insanely fast.
You'd want to look into tutorials regarding "Shader Rendering to Texture".
Someone built "the Game of Life" into a texture (using every pixel!) and rendered it to a torus.
And that's about as per-pixel as you can get.
One more possibility. Thinking about this from the time axis. Assuming you're talking about that map and scanning every piece on a chunk. How much of that map needs to be updated in real-time? Can a particular chuck update be "deferred" across multiple frames? I can't remember, does the map show live movement of "alien attacks"? If not, a "chunk parse" could certainly cross a few frames and still "appear" instant to a user if that one piece is dragging the FPS down.