How to reproduce:
1. Download and open user's attached project "UnityMaterialEditorBug.zip"
2. Select the "Editing Values Extremely Laggy" material, and try editing any value there(notice that the value changes with a slight/medium lag)
3. Delete the "LotsOfTypes.cs" script which contains 100 000 empty regular c# classes and notice how this problem goes away

Expected result: The amount of managed types in the script doesn't have any impact on the MaterialEditor inspector's performance
Actual result: The MaterialEditor inspector becomes increasingly laggy the more managed types you have

Reproducible with: 2018.3.0f1, 2018.3.5f1, 2019.1.0a11
Not reproducible: 2017.4.17f1, 2018.1.9f2, 2018.2.19f1, 2019.1.0b3