Reproduction steps:
1. Open the user's attached project "VisualScripting.zip"
2. In the Project window open Assets/Scenes/SampleScene
3. Go to Edit/Project Settings, select Regenerate Units and observe how much time it takes to finish
4. In the Assets folder select "New Script Graph" asset and in the Inspector window select Edit Graph
5. In the Project Settings select Regenerate Units

Expected result: Regenerating Units with an open Script Graph window takes the same amount of time as with a closed Script Graph window
Actual result: Regenerating Units with an open Script Graph window takes more time than with a closed Script Graph window

Reproducible with: 1.4.13 (2018.4.30f1, 2019.4.17f1, 2020.1.17f1, 2020.2.1f1), 1.5.1 (2021.1.0b1)

Notes:
- Whenever custom nodes are adjusted, rebuilding the type graph via Regenerate Units takes longer when a Script Graph window is open. The issue reproduces inconsistently and depends if the nodes were adjusted before
- The issue will reproduce when the project is opened for the first time, however, the issue might not reproduce when constantly adjusting the nodes
- The issue is reproducible with an open State Graph as well
- The user noted that it takes approximately 45 seconds to Regenerate Units with an open Script Graph window and approximately 15 seconds with the Script Graph window closed