Add threshold for RSpec tests levels in testing guide

doc/development/testing_guide/best_practices.md

@@ -358,6 +358,19 @@ a place to start. The most expensive examples here are in
 shared examples; any reductions generally have a larger impact as
 they are called in multiple places.
 
+#### Top slow tests
+
+We collect information about tests duration in [`rspec_profiling_stats`](https://gitlab.com/gitlab-org/rspec_profiling_stats) project. The data is showed using GitLab Pages in this
+[UI](https://gitlab-org.gitlab.io/rspec_profiling_stats/)
+
+With [issue](https://gitlab.com/gitlab-org/gitlab/-/issues/375983) we defined thresholds for tests duration that can act a guide.
+
+For tests that are not meeting the thresholds it is recommended to create issues and improve the tests duration.
+
+| Date | Feature tests | Controllers and Requests tests | Other |
+| --- | --- | --- | --- |
+| 2023-02-15 | 67.42 seconds | 44.66 seconds | 76.86 seconds |
+
 #### Avoid repeating expensive actions
 
 While isolated examples are very clear, and help serve the purpose of specs as