So the problem is occurring because a single `Test` class is being instantiated with three different tests here in datetimetester.py:

            for name in ZoneInfo.zonenames():
                Test = type('ZoneInfoTest[%s]' % name, (ZoneInfoTest,), {})
                Test.zonename = name
                for method in dir(Test):
                    if method.startswith('test_'):
                        tests.append(Test(method))

here: https://github.com/python/cpython/blob/master/Lib/test/datetimetester.py#L4853

The `Test` class which is being dynamically created is being instantiated with the test methods: test_folds, test_gaps, test_system_transitions and is being appended to tests. This makes that class to appear thrice in `test_classes` in `test_datetime.py`:

        elif issubclass(cls, unittest.TestSuite):
            suit = cls()
            test_classes.extend(type(test) for test in suit)

here: https://github.com/python/cpython/blob/master/Lib/test/test_datetime.py#L34

Hence, the class gets `_Pure` and `_Fast` appended to its name thrice and gets executed thrice, making the tests take 3 times as long to run.

This is confirmed by changing the code to the following:

           for name in ZoneInfo.zonenames():
                Test = type('ZoneInfoTest[%s]' % name, (ZoneInfoTest,), {})
                Test.zonename = name
                tests.append(Test())
                # for method in dir(Test):
                #    if method.startswith('test_'):
                #        tests.append(Test(method))

However, I'm not sure what implications this has w.r.t. unittests and the advanced cases.

The other way to fix it is to create a set out of the classes, as suggested in PR #2534.

~
ut