Once a map gets built up significantly, the GPU can start struggling to draw all the graphics, causing lower frame rates or draw lag.
It's all about visible faces. A face is the side of a block. If the side of a block is exposed to air, that side is a visible face and the game must draw it. The more visible faces the game has to draw, the more work it has to do; the more likely the map will start to hit the limits of the graphics hardware.
1. Hidden visible faces. How can visible faces be hidden? These are faces inside enclosed areas. The game does not know if a face is inside an enclosed area, all it knows is if the face is exposed to air. Significant landscaping can create enclosed areas. e.g. if you build a flat surface over a non flat terrain, this can leave empty areas underneath the flat surface you built, down to the ground. Any face inside this enclosed area will still have to be drawn, even though players will never see them. So to remove these hidden visible faces, fill in the enclosed areas. The Creative Fill tool is a great way to do that.
2. While the sunrise/sunset effect is active, the GPU is executing a lot of extra commands to create the effect. If you freeze the day/night cycle so that the sunrise/sunset effect is always active, the GPU is working much harder than it is during normal day or night time. If your map is experiencing draw lag, freezing the day/night cycle outside the sunrise/sunset effect can help.
