Prerequisites:
For repro it's needed to have the Fontainebleu demo downloaded on your machine.

2019.1 version: https://github.com/Unity-Technologies/FontainebleauDemo
2019.2 version is on Perforce: https://confluence.hq.unity3d.com/display/DEV/HDRenderPipeline+-+Fontainebleau+Demo
Perforce server is p4d-par-p01:1666
User: "guest" pass: "UnityGuest"

Steps to repro:
1. Open the attached project for 2019.1 in the latest 2019.1/staging version;
2. In the context menu select Load levels->Day;
3. In the Hierarchy double-click on the Demo_LI_Day scene to make it a master scene;
4. In the Lighting settings click 'Generate Lighting' button;
5. Wait until the lighting is baked and note down the Total Bake Time and Bake Performance from the bottom of the Lighting window;
6. Save and close the project;
7. Open the 2019.2 version of the project in the latest 2019.2/staging build;
8. Repeat steps 2-5;
9. Compare the bake time and performance of the 2019.1 vs 2019.2 builds.

Expected result:
Bake time and bake performance are almost the same.

Actual result:
Bake time and bake performance in 2019.2 is ~20x slower than in 2019.1. See the numbers below:

Version 2019.1.14f1 (148b5891095a) Branch: 2019.1/staging:
Total Bake Time: 01:56:45
Bake Performance: 0.19 mrays/sec

Version 2019.2.2f1 (708b70bafd23) Branch: 2019.2/staging
Total Bake Time: 23:23:25
Bake Performance: 0.01 mrays/sec

Regression introduced in Version 2019.2.0a3.
Tested on the Intel Core i7, 16GB RAM and GeForce GTX1070.

Notes:
- Reproducible in 2019.2.2f1;
- Not reproducible in 2019.1.14f1;
- Could not reproduce in 2019.3 or in the trunk as the project is not supported for these versions;
- Reproducible only in both Fontainebleu project and in the small test project (that also shows the performance degradation, but not that big as in the bigger production project);
- Reproducible on Windows 10;
- The issue seems to be isolated around the LOD groups with the LOD0 being lightmap static and the scale in lightmap set to 0
- There are following symptoms of the slow bake in 2019.2: Initially after baking starts the ETA is shown as 10 hours and it slowly decreases over time, after ~8 hours the ETA starts to jump up and constantly increases. It looks like the baking is stalled, but it finally bakes after almost a full day of baking.