Steps to repro:

1. Open attached project and 'Bedroom_modelling' scene;
2. Drag 'Bedroom_Lighting' scene on top of 'Bedroom_modelling' scene in the Hierarchy;
3. Double-click on 'Bedroom_Lighting' scene in the Hierachy to make it active;
4. In the Lighting window select 'Progressive - GPU (Preview)' in the 'Lightmapper' dropdown list;
5. In the Lighting window press 'Generate Lighting' button;
6. Wait until scene bakes.

Expected result:
Scene bakes relatively fast and Editor is not unresponsive while baking.

Actual result:
Editor is almost unresponsive and the bake performance is 0.17 mrays/sec on 1070GTX (40x times slower than CPU PLM).

Notes:
- Reproducible in 2019.1.0a12, 2018.3.1f1, ;
- Reproducible on Windows 10 with NVIDIA GeForce GTX1070 (8GB VRAM);
- The issue is reproducible only with GPU Lightmapper; not reproducible when baking with CPU Lightmapper or Enlighten.