I used the original 2020.3.xx Unity editor and upgraded the existing project to 2021.2~2022.1b Unity editor.
The animations I was working on were about 40 frames long, and I was using a rig with a complicated hierarchy.
And after locking unnecessary properties, after importing Animations, I tried to export about 10 Anim files as they are.
Exporting seemed to be done normally until the second Anim, but after that, a message like 'Waiting for script to be executed..?(should've captured..)' appears and the export starts to take a very long time.
And if I downgrade the same project to 2020.3.xx, this exporting issue disappears.
I did a similar test on several similar projects, and exporting was too slow on all [2021.2.8, 2021.2.11, 2022.1.0b8 Unity editor].
I'm not sure if this was an issue that only happened to me and Since I can downgrade the Unity editor anytime so it's not an urgent issue.
However, I think this is an issue worth checking out, so I am writing this report.
Customer support service by UserEcho