Just to illustrate, some pretend-analogous run time type-inspect based dispatch code in python (a language not known for its blazing speed) is still much, much faster than this:

   In [3]: timeit a = [1];  a.__len__() if type(a) is list else len(a)                                                                                                                                       
   126 ns ± 0.615 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)