Write fast unit tests

As mentioned before, fastness is one of the characteristics of a good unit test. Try to make your unit tests fast enough. Slow unit tests may discourage developers to run them as frequently as they should, especially when they change code, which can lead to regression. In addition, when unit tests are part of a CI/CD pipeline (which is a good thing btw), they can slow the whole process.

To be more precise, and this is my opinion, a unit test should not take more than 500ms to run. Any test that takes a long time to be run should be reworked.

When a unit test is slow, most likely it is doing something it shouldn't be doing like accessing the file system or an external resource.