It's not a very well optimized game :3

There's no LOD system so stuff far away is rendered at full detail, and it also renders stuff you can't see like caves, on top of running primarily on a single core, so once you try to play on higher settings, with shaders or mods, or when you build big builds with lots of entities, like if you have farms or use a lot of item frames or armour stands the game really starts struggling to keep up :3.. there are optimization mods for it tho, which makes the performance a lot better (with sodium + lithium I can usually increase my FPS by 200-300%)

Java most likely