Lag and glitches when another player joins world

I’m experiencing a very annoying bug related to lag. When I’m playing in my own world, the game runs perfectly fine. But whenever my brother joins my world — we live together in the same house — everything starts glitching. Mobs and zombies begin teleporting, and when you break blocks, they take a long time to drop items.

So basically, every time someone joins my world, or when I join my brother’s world (the same thing happens there), everything starts lagging and glitching. It’s a really frustrating problem.