It's quite easy to have slow tests that aren't integration tests. For instance, there's some tests in Sympy that are only a few lines of code that run very slow

It's quite easy to have slow tests that aren't integration tests. For instance, there's some tests in Sympy that are only a few lines of code that run very slow because the calculation is difficult. Sometimes (but not always), it's trying to calculate a very difficult integral (which is a test of integration, but not an integration test).