Vanilla Minecraft can run smoothly on old computers but when you add a shader pack to the mix, it can cripple your FPS. The problem that shader packs pose is that they can be very taxing when it comes to game performance.
For better lighting, sky, and cloud textures, you need to look at shader packs. While the vast majority of them focus on texture resolution and details, lighting is rarely touched. If you dig deep enough, you can find Minecraft resource packs that make it look like a whole different game.
