This was detected by our performance tests:

[https://observer.prd.ds.unity3d.com/stonks/Unity.GI/variations/editor/pivots/editor/changesets/6a05ce51ff3c] 

The commit range to look into is between `d832162ee654ad4fc1c34ab4dfe55986670f5b06` (good) and `ba913132852ff450c8508fac2f47e7a454c8b7c6` (bad). 

 
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How to reproduce:

On a Windows machine with a modern GPU – 

Using the project under unity/unity: 
# Open the test project Tests/PerformanceTests/GI
# Run the UnityEditor.GI.Tests.Performance.LevelsOfDetail.ManyInstancesWithLODGroupWithLOD("0",ProgressiveGPU) test. 
# Note the Bake Time Total and Bake Performance under the performance test report window.

You *will* run into compilation errors in the bisection commit range (which is why we haven't been able to do it automatically). When you do, apply the following changes to the project: [https://github.cds.internal.unity3d.com/unity/unity/commit/cb5e3327b7469c6f15206f50e026ccd530daf13c] . You can just copy the entire file from here [https://github.cds.internal.unity3d.com/unity/unity/blob/cb5e3327b7469c6f15206f50e026ccd530daf13c/Tests/PerformanceTests/GI/Assets/Tests/PerformanceTests.cs] and replace the existing Tests/PerformanceTests.cs.

Note that I can't provide expected / actual numbers as they depend on your hardware. The regression is that Bake Time Total is about 4-5 times larger and Bake Performance is equally lower than before. See the Observer link above.