Speed is not a matter of choosing a certain language, speed is a matter of choosing the right algorithms.

Apart from that, writing whole programs in assembler is not recommended on modern operating systems because you would have to program against a C-level interface. But nothing (except for portability) speaks against writing small functions (e.g. for image processing) in assembler. But don't get the illusion that asm is faster than C. Most C-compilers nowadays produce optimized asm code that is faster than anything you would write yourself in asm.
I'd also suggest to skip C++ and learn Java instead. C++ is (or should be) a dead language (although many people don't want to believe so) and not a good choice for object-oriented programming. In other words, C++ is a mess and mostly incompatible with itself.
Python OTOH is a really nice and comfortable object-oriented language.
C is tiny, easy to learn, but it's not really comfortable for writing big programs.