In step 9 of 9 of the repo 2 to 3 migration process ( Upgrade Agent User Interface ), the migration process schedules and then runs synchronously a task that rebuilds the search indexes of repository 3. Completion of the migration process is blocked for the end user ( cannot click "Done" ) until this process completes. Depending on the amount of content to index this process can take hours.
The primary advantage of blocking migration completion by this task is that when the task is finished, the users of repository 3 can be assured that Search operations for content, and the REST APIs related to them will be available and working.
If actually having the search indexes for content working immediately on migration end is not considered critical to considering migration "complete", then there should be an option to save this hours of time and instead perform the search indexes building asynchronously to the migration process.
Implement a feature to build search indexes for All repositories asynchronously as the last step of the migration process and not block an end user from marking the migration as "Done". Note: this is with a non-UI solution