Reproduction steps:
1. Open the user's attached project
2. In the Hierarchy right-click "School" Game Object and choose "Convert To FBX Prefab Variant"
3. In the Convert Options Window press "Convert"

Expected result: Game Object is Converted To FBX Prefab Variant in a few minutes
Actual result: Game Object Conversion To FBX Prefab Variant can take up to 45minutes

Reproducible with: 3.2.1-preview.2 (2019.4.37f1, 2020.3.32f1), 4.0.1 (2019.4.37f1, 2020.3.32f1), 4.1.2 (2019.4.37f1, 2020.3.32f1)
Not reproducible with: 4.1.2 (2021.2.17f1, 2022.1.0b14, 2022.2.0a9)

Notes: The object gets converted, but The final Prefab Asset does not have material.
With 2021.2.17f1, 2022.1.0b14, 2022.2.0a9 the process takes under 10 minutes.