Why does only one world in minecraft lag so much?

So all of my worlds on minecraft work just great but one of them is laggy as can be. And it's always like that for that world. Its in the rain forest if that helps. Why is it acting like this and how can i get it to stop?

Why can I only play the Minecraft demo?

So I was going to play Minecraft but it says I can only play the demo. (It's not like I have been off for a long time, I was on like the day before it happened.) I don't log in every time I play it because it always says I'm logged in. I have tried to fix it but I can't. It's getting annoying and I don't want to play the demo forever. Do I need to buy it again? Can someone help?

Games similar to ROTMG (Realm of the Mad God)?

Does anyone know any games like rotmg? I like toting, but hate the permanent death feature. I like feeling accomplished when I do something, but in toting I feel like I can't do anything due to the perma-death. I do not really like side strollers though… I do like the pixelated graphics and basic controls. I also have played minecraft so don't say that.

Good desktop in the $300-$400 range? (read details)?

So I'm looking to get a new desktop, and I'm just looking for some opinions on some good ones for some games, such as minecraft, skyrim, titanfall, etc. I think the highest I would go is $500 maybe. I'm not looking for some fancy gaming computer, just one that can support these games, and some others.

Any free, safe, and headset compatible recording software for Mac?

I have a YouTube gaming channel and I need some good and free recording software for Minecraft on my Mac. I know about QuickTime Player, but I need to use my headset, and when I do, it only picks up my voice and not the game sounds. Like I said, it needs to be free, safe, and something that doesn't have a logo on the screen. NO EXEPTIONS FROM THAT Please. But, if you knew how to make Quicktime Player work the way I need it to, that would be great too. Thanks!

Youtube Channel: www.youtube.com/PenguinPancakesGames

Why do these giant holes form in Minecraft?

In my Minecraft worlds there have been giant holes that just form out of nowhere and cause a TON of lag. Everyone that joins my world (its a LAN world) can see it and are affected by it too. No one can go into it and it just teleports you back to where you were before you tried to go into it. Not only that but about 40 creepers spawn in bulk in the hole and fall into the void. Its literally 40. I've had to delete countless GOOD worlds because of it. Please help!

Added (1). I have tried logging out and back in and they are still there and in the same place. I have also tried going to the nether and back.

Minecraft online won't work?

So I want to play Minecraft online, and it told me I needed Java to play so i downloaded Java but now it's telling me that the deployment rule set won't allow it to run. How can I change this? I really want to play.