It is very important that all the tests are run and not only the last test that was written. The code that we just wrote might have made the last test pass while breaking something else. Running all the tests confirms not only that the implementation of the last test is correct, but also that it did not break the integrity of the application as a whole. This slow execution of the whole test suite is a sign of poorly written tests or having too much coupling in the code. Coupling prevents the easy isolation of external dependencies, thus increasing the time required for the execution of tests.
In this phase, we are in the green state. All the tests are passing and the application behaves as we expect it to behave.