How to reproduce:
1. Open attached project "test.zip" and scene "SampleScene"
2. In Audio Mixer window, select "main" mixer
3. In Audio Mixer window, select "main2" mixer

Expected result: the transition between different mixers is relatively fast
Actual result: the transition between different mixers is slow

Reproducible with: 2019.3.0a1, 2019.4.6f1, 2020.1.0f1, 2020.2.0a20 (asset import pipeline version 2)
Not reproducible with: 2018.4.26f1, 2019.3.0a1 (asset import pipeline version 1)

A few examples when the issue is visible:
1) When editing a prefab selecting it in the project view. You edit a field and then the Import Kicks in.
2) When changing names of a file in the project
3) When cloning things In the project, like for example a material
4) When selecting audio mixers in the Audio Mixer window