A bottle neck is, essentially, a spot within a path in which the flow of something slows down.
Think of it this way: You have a pipe the diameter of your arm, so water flows through it at a certain speed.
Then you have a part of the pipe which has the diameter of a BOTTLE NECK.
In that part of the pipe, the flow of water will slow down.
In terms of computing, it is a part of your system, whether physical hardware or in the software, in which data flow rates slow down. In UZO's case, the physical problem of the read/write speed of a conventional hard drive causes a natural bottleneck. Thus, he switched to a SSD (Solid State Drive), which, by nature, has no such physical problems. Thus, he solved the bottleneck situation and as a result has a faster system/server/MinecraftWorld.
I'm interested in Minecraft. I'd like to build something for it but I don't want to spend time fighting zombies or whatever: I just wanna build.
Maybe something nice and Castlevania-ish.