Minecraft already runs fine. Making it 20x faster is great, but beyond that, you're running into PCMasterRace type optimizations that only 6 people in the world can see.
Minecraft only runs fine on the fastest hardware. I've got an i7-4790k that runs at 4.4GHz for single-threaded tasks. Just flying through a minecraft world in a straight line in creative mode needs 80% of one core to keep pace with the world generation. No laptop can run the game smoothly as-is. That 20x performance improvement is needed just to get into the territory of reasonable performance on the kind of low-end hardware kids typically have available.
People don't spend most of their time in-game that way, but they definitely do it. Especially when looking for an interesting places to set up.
Multiplayer gets much worse though, god forbid you have several people exploring new terrain on a server.
And god forbid you play with any of the mods that add faster railways[1] or jetpacks[2] or boots that make you walk faster[3], etc. It's a bummer that the optimizations are coming to the less moddable version, MC is a great platform for people to build new gameplay on.
