This is during the migration with mix ecto.migrate?

Check htop. Are you using all your RAM? If not you should ramp up the database settings. In particular edit postgresql.conf and set work_mem to a higher value. Mine is 16MB.

Also if Pleroma is running, you should definitely stop it. I see no reason dropping the index should take a long time unless the database has locks. The thing that takes a long time is creating the new index afterwards.