How can one claim that an interpreted language is fast?
Easily. If you implement the language using itself and the resulting interpreter is faster than the original then this means that the interpreted language manages to imprint the prorgammer’s intentions to the C language (or assembly) better than the programmer herself.
For example, I may create an interpreted language named V using C and try to make the implementation as fast as I can. Of course this means that I have to implement a number of data structures and handle them. Also I have to optimize code path etc etc.
Then I re-implement the V language using the V language itself. If the re-implementation if faster than the original implementation then this means that the interpreter’s logic produces faster code than me.
Well… here it is. The Python implementation in Python is faster than the Python implementation using C!
Read: It is very probable to end up with a faster program if you write it using python instead of C because the language will do better than you will in optimizing your data structures and your code. Just like it is better to write C than assembly and let your compiler to produce the optimized assembly code.