Upgrade from v13 to v14 - Decrease time for batched migrations

We are about to upgrade from v13 to v14. (13.12.15 to 14.0.2)
Doing some tests, batched migrations executes for almost 12 hours. Documentation warns
that this could happen for some installations when upgrading to v14, but also says : “Decrease the time required to complete these migrations by increasing the number of [Sidekiq workers]:”
We tried configuring trhee workers like this:

sidekiq[‘queue_groups’] = [
"background_migration ",
"background_migration ",

That is, one worker for all queues and other two workers for migrations. (We made sure there are enough cores for them).
Then we made the test again, and it took the same time for batched migrations. Also, when looking progress they execute sequentially (shouldn’t execute in parallel because of more workers?)
Any idea why this isn’t working ?