I get solid framerates, generally between 65 and 90 fps, but water flows slowly, blocks sometimes get 'unplaced' or placed twice, it takes like 3 seconds to throw an item, same to pick them up, chunks often refuse to load and mobs move at around 1 frame per second. It also takes around a minute just to load the world once I've started up the game. I don't know exactly when this started happening, somewhere between yesterday and the day before that. It's the same in both the newest snapshot, an older snapshot, 18w47a, and 1.13.2. Minecraft is only taking about 25% of my ram usage, so I'm not sure what's wrong with it.