C is absolutely not faster than C++. Generic types in C are typically done through type erasure or an intrusive type. C++ templates explictly allow different instances of code to be generated for each of the types. So, the code can be specialized for each type, which most of the time is faster. In general, template metaprogramming allows for drastically more complex optimization at compile time.

The biggest reason to write in C over C++ is simplicity and higher level of control over algorithms and data structures. Portability is another reason though.