LiveServerTestCase slow

Approximately 1 time out of 4, when I try to run my functional tests, I've to wait like 20 minutes for my tests to start :

python manage.py test
Using existing test database for alias 'default'...

And this issue is not happening when I run my unit tests.

Even when I try to run simple tests like :

class FrontTest(LiveServerTestCase):

        @classmethod
        def setUpClass(self):
        super(FrontTest,self).setUpClass()

        @classmethod
        def tearDownClass(self):
        super(FrontTest, self).tearDownClass()

        def test_stupid(self):
            print('hello world')

Does this problem ever happened to you ?