Python is excellent for bitcoin apps.
What makes is so good is that it already has good easy to use libraries for doing all the math behind bitcoin.
Support for: big numbers, SHA256, ripemod160, elliptic curve operations etc...

Those can be a real PITA if you have to write any of them in C++, or whatever.
Actually C++ also has some libraries you can use, but at least those that I have seen are quite difficult to use and complicated.

On python being slower... yes, compiled languages are faster, but you wont be needing to use the full capabilities of a computer for doing bitcoin operations. 

One example of a python program is Electrum