is there a reason to not use cython everywhere?

Question

I learned cython an hour ago, i changed ~10 lines out of thousands of lines in my code and i already got an absurd speed improvement on something that wasn't even my main bottleneck according to the profiler (but still called millions of times).

And the cython report still show pretty much everything in the cythonized code in yellow. (which mean it's not optimal, yet, as far as i know)

this is what i changed, everything else is still "as-is" :

@cython.freelist(32)
cdef class Vector:
    cdef public double x, y

    def __cinit__(self, double x=0, double y=0):
        self.x = x
        self.y = y

    ... (many other methods i didn't touch yet)

is there a reason / use-case to not use cython everywhere and everytime you create a class ?