Minimal repro without generics and third-party deps:

from typing import overload

@overload
def foo(a: bool, b: bool) -> bool: ...
@overload
def foo(a: int, b: int) -> int: ...
@overload
def foo(a: float, b: float) -> float: ...
@overload
def foo(a: str, b: str) -> str: ...
@overload
def foo(a: bytes, b: bytes) -> bytes: ...
def foo(a, b):
    raise NotImplementedError
    
foo(10, foo(11,
    foo(foo(foo(foo(foo(foo(foo(foo(foo(0, 1), 2), 3), 4), 5), 6), 7), 8), 9)
))

This takes 6 s with compiled mypy 1.15.0. Adding one more term increases runtime to 18 s. The dependency (time vs number of terms) is $T \approx 3^N$ after 7-8 terms. For every relevant overload signature (3 out of 5 here), we accept all arguments. When we have 10 such overloads nested, we accept the "leaf" nodes 0 and 1 $3^{10} \approx 60,000$ times.

np.ndarray.__mul__ is an overload with 17 variants, 9 of which match NDArray[np.floating] argument, so it grows even faster.