LINQ to SQL - The big threat

LINQ to SQL seems magical at first sight. You can query over your objects without worrying about how database stuffs happen under the hood. That's where threat lives. Because Linq late processing behavior without your noticing your 3 tier application is not 3 tiered anymore. All you did is to encapsulate and spread data access logic throughout all your application tiers.
Imagine you have a huge system and it suddenly encounters a database performance issue. Then you see there is a bad sql statement reaching database. Where is it in your system as sql and linq statements are not necessarily the same?
Remember: Entity framework relies on linq to sql.