Be careful when you add users explicitly to SharePoint Groups - it's not recommended approach by several reasons
- SharePoint ACL is limited to the number of users. For the large farms your might hit the limit of user's you can add.
- New added user forces Search to perform "Security only Crawl" full content recrawling (that cause performance issue).
If you add/remove users from the SharePoint group it will force search to crawl content - "Security only crawl". Incremental crawl starts to update security changes, and all “Updated ACL’s” must be pushed down to all affected items within the index. This process might take a lot of time if changes affect a lot of items in SharePoint and you have big number of users in SharePoint.
Managing users via AD groups won't cause ACL changes, and no security only crawls will occur.