I saw in the announcements that’s he disabled the rain weather effect, that is causing this huge CPU usageQuote:
Unfortunately the performance is so freaking high making a 2.60GHZ laptop from 3% cpu usage to freaking 30% fans are working like crazy. Other than that nice nostalgia, mobs are not moving right they sometimes jump in areas, like in emulators. You might disconnect at times when picking up items and so on..
Hopefully the administrators will take time to fix bugs and release the server again once it's done right..