C ++ is a defacto language not only for game development, but also for everything else, including the browser that you use with the OS it runs on.
So, if you are considering the seriousness of programming, C / C ++ should be at the top of your list.
As for speed, C ++ is generally faster than Java. Why at all? Well, the truth is that C ++ is faster than Java, faster than almost everything else in fact, but sometimes C ++ code can be poorly written, and Java can prevent beginners from making some mistakes, resulting in many claims about that Java, like C ++, there are even claims that Python / Psyco are as fast as C ++ and faster than Java.
Keep in mind that no program written for the framework (Java, .NET, VB, etc.) will ever run faster than a program running at a lower level (directly on the OS or even on the hardware), e.g. in C / C ++, ASM, Pascal, etc.
Take a look at this:
Poorly written C ++ half the speed of Java, correctly written much faster
At the end of the day, you should go with a language that will be comfortable with you, remember that python is ready to work, commercial, stable, great for beginners, awesome for use in Web 2.0 projects, not to mention that too very funny.