It is pretty well optimized. I think it might not be, like, genius-level amazing, but the devs care about performance and worked to improve it.

In the end though, it’s a game where the entire map (as generated so far) is simulated - I think there’s cases where chunks go to sleep, but it’s not Minecraft’s “stop simulating anything not next to a player”. When combined with players building lots of machines moving many, many items around, you’ll inevitably end up with some serious CPU usage. Not a problem on a decent computer, but I have had friends struggle on weak laptops, even getting dropped as they literally couldn’t keep up with the server.