The Dynamic setting for the Nav Mesh generation causes a large dip in frame rates

When using a fairly large level the Dynamic setting for the Nav mesh generation can cause large drops in frame rates. (Example: 120 fps when running Dynamic with Modifiers only to 42 fps with Dynamic)

1. Open the attached project
2. Click play in Editor
3. Press the '~' key
4. Type in the command Stat FPS
5. Notice the frame rate is high
6. Stop PIE
7. Change the the Nav mesh generation to Dynamic
   Project settings > Navigation Mesh> Runtime Generation > Dynamic
8. Start PIE
9. Press the '~' key
10. Type in the command Stat FPS
11. Notice a large difference in frame rate

Expected: Drop in frame rates should not be so dramatic
Results: Loss of around 60+ fps