Adding roles to group with a lot of user inside the group causes long response time

[tung2744]

1. We have a group with ~20000 users
2. Add a role to that group
3. See long response time

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Guessing the time was spent on updating the user's require_reindex_after column, or enqueue tasks to reindex the users.

• [ ] Find the reason causing the long response time.
• [ ] If it is related to enqueue tasks, see if we can do it after returning the response.