Which technology should I use to develop a high performance web application
I kinda think this is almost more like a religious problem, than a real technical issue. For almost every programming language you can find a big website that's using it.
.NET -> Microsoft
Ruby -> Twitter (yes, they have a few issues, but still)
PHP -> Facebook
Java -> Lots of finance companies
Don't know about Phyton, but I'm sure there is.
More important is a good scalable architecture. That is where Twitter kinda screwed it up it seems. 
Personally I use ASP.NET. Works fine, is somewhat easy and has a nice IDE. And the market is not so fragmented. Before I used Java with Websphere. Was running on a Sergenti Sun Box, so could definitely handle a lot.
I would more see into what you can get yourself into the quickest. If you know C++ C# or Java are easy to learn.
I have couple of ideas in my brain which I would like to bring out before it's too late. Basically I want to develop a web application which I could sell it to clients. So which technology shall I use to accomplish this? I have been a C and C++ software developer but it's been a very long time since I have developed one. So the things I would like to know is:

Scalability and Performance?
Easy way to develop web application in a faster manner?
Any Framework?
Application server?
and which programming language?