A 2x performance difference means 1000ms instead of 500ms to load a 
     page ... It's also the difference between 30 fps in a game vs. 15 fps - that 
     is really noticeable.

You don't know what you're talking about ...

In web apps it really depends on where your bottleneck is. A 2x performance difference could mean just a 5% difference in response time.

On the other hand, go ahead and try handling 100.000+ connections on a single server from C/C++, like these dudes right here ... http://amix.dk/blog/viewEntry/19456

Of course it can be done, but you'll need to read a whole book about it, and at the end of the day your app also has to do something useful other than returning "Hello".

And as game development goes, game developers have been using high-level languages for the business logic for years, including Lua, Python, Java and C#.

     The responsiveness is also part of what attracts users

True, but I'll bet I can get better responsiveness in Java than you can in C/C++.