How to reproduce:
1. Open a big project (tested with 1.5GB)
2. Open the Test Runner
3. Switch between EditMode and PlayMode tabs
4. Notice some delay while switching tabs
5. Create new project
6. Repeat steps 2-3
7. Notice, that there is no delay while switching tabs

Actual result: There is a small freeze when switching tabs (the freeze depends on project size)
Expected result: There should be no freeze when switching tabs.

Reproducible with: 2018.1.0f1.
Not reproducible with: 2018.2.0b2, 2018.1.0f2, 2017.4.2f2, 2017.2.2p4.