Comment by ant6n

You need to do mid-frame tile updates just to show a full bitmap frame. There’s 360 8x8 tiles on the screen, but the tile indices are 8 bit (you can only reference 256 tiles). You can store only 384 tiles in VRAM - a bit more than a full screen. So the mid-screen update is to go from one tile dictionary to the other, so you can access 360 tiles in total.

You can update 1 tile per scan line (during hblank), so 154 tiles per frame (including 10 vblank scanlines). So you need 2.5 frames to replace all tiles.

If you are really smart about updates, you can “race the beam”, basically start updating tiles just as the frame starts rendering, just behind the active scan line. Then you can update maybe 280 tiles before the active scan line of the next frame catches up with you.