From this forum post:
https://forum.unity.com/threads/asset-pipeline-v2-script-reimport-performance.784322/#post-6134054

It appears that OnValidate is now called every time a Monobehaviour that belongs to a Prefab changes with AssetDatabase V2.
With V1 this does not happen.

Repro with V2:
2020.2, 2020.1, 2019.4

V1 doest not call OnInvalidate()

Expected:
ADBV2 should not call OnInvalidate() for every single prefab when a script changes