Editor taking long to open with no log output

After upgrading to 2019.4.0, the editor takes 5+ minutes to open (previously < 1 minute on 2019.3.9) with no log output for at least 3-4 minutes. Here's the line in the log before the wait...

" Launched and connected shader compiler UnityShaderCompiler.exe after 0.07 second "

And here's the log after 3-4 minutes...

"
Initializing Unity extensions:
'C:/Program Files/Unity/Hub/Editor/2019.4.0f1/Editor/Data/UnityExtensions/Unity/UnityVR/Editor/UnityEditor.VR.dll' GUID: 4ba2329b63d54f0187bcaa12486b1b0f
Can't find custom attr constructor image: C:\docs\AoLUnity\AshOfLegends\Library\PackageCache\com.unity.ads@3.4.5\Editor\UnityEditor.Advertisements.dll mtoken:
0x0a00000f due to: assembly:C:\Program Files\Unity\Hub\Editor\2019.4.0f1\Editor\Data\Managed\UnityEngine\UnityEngine.CoreModule.dll type:UnityAPICompatibili
tyVersionAttribute member:void {0}(string,string[]) signature:.ctor
LightmapEditorSettings: switching bake backend from 1 to 2. "

 

Every time. Like clockwork. Task manager shows no activity other than unity taking 17% cpu and not responding.

 

So I fixed this by deleting all the folders in the Library folder. This may sound easy but it took several hours, essentially re-importing everything twice.