class Foo:
      __slots__ = ("a", "b")
      a: int
      b: float

* a user can define subtype `class my_int(int)`, so you cannot optimize the layout of `class Foo`

* the builtin `int` and `float` are big-int like numbers, so operations on them are branchy and allocating.

and the fact that Foo is mutable and that `id(foo.a)` has to produce something complicates things further.