The following errors (compiler-errors) show up whenever the Entities package is (re-)imported.
Library\PackageCache\com.unity.entities@0.0.12-preview.30\Unity.Entities.PerformanceTests\EntityManagerPerformanceTests.cs(198,10): error CS0246: The type or namespace name 'PerformanceTestAttribute' could not be found (are you missing a using directive or an assembly reference?)
Library\PackageCache\com.unity.entities@0.0.12-preview.30\Unity.Entities.PerformanceTests\JobForEachPrefilteringPerformanceTests.cs(79,10): error CS0246: The type or namespace name 'PerformanceTest' could not be found (are you missing a using directive or an assembly reference?)
Since they’re just tests, deleting the Unity.Entities.PerformanceTests-Folder is enough to get rid of the compiler-errors. However, I’m also not able to create my own (ECS-)Tests (as the namespace cannot be reached for some reason).