Unity responsiveness is severely limited while Progressive Lightmapper is preparing to bake lighting. On some systems, freezes occur lasting for several seconds

How to reproduce:
1. Open Unity
2. Load user-submitted project (test MODELS.zip)
3. Load scene 'Demo1.unity'
4. Select Progressive Lightmapper in Lighting settings
5. Press "Generate Lighting" - observe Unity response time while 'Preparing bake'

Expected result: Unity maintains responsiveness throughout the light-baking process.
Actual result: Unity responsiveness is severely impacted and sometimes freezes occur.

Reproducible with - 5.6.0b6, 2017.1.3p1, 2017.2.1p3, 2017.3.1f1, 2018.1.0b6