User Profile is one of the SharePoint features, which provides searching across your users and surfacing user's info across your organization. Such scenario is very common in intranet scenarios.
To implement such requirement user's information must be imported to SharePoint first. Unfortunately it's very slow process, because during importing a lot of background tasks are run. For the large farm you need to plan enough time to perform user's profile import, it's not what can be done for 15 mins.
Profile database import takes roughly an hour for every 200 profiles.
Update: In Microsoft’s 2 million user test, it took 3 days to crawl to crawl Active Directory and populate the profile database! This goes a long way into illustrating why you’d need to budget for extra time in your plan. Now imagine making a mistake and having to re-do an import!