Upgradingfrom django 1.11 to 2.0.8 python unittests became longer from 18 to 28 minutes

I am updating project which is build on django 1.11 version to django 2.0.8 version and I noticed that backend tests are taking much longer. It is taking now 28 minutes (instead of 18 minutes that was in django 1.11).

Do you have any ideas what may cause that problem, because it is crucial for the project to keep the time of the tests less.