Enforcing database integrity
Truth be told, not only will you not see much performance loss from having foreign key constraints in the database, but you will see performance enhancements. The SQL Server query optimizer is built around the concept of primary and foriegn keys as well as other types of data constraints. If these are in place and enforced the optimizer can take advantage of them to get you better performance. Here's a blog post with a simple example that shows it in action.

If you are in an edge case where you truly have more inserts than reads (and updates &amp; deletes require reads, so they usually end up adding to the read count), then it might make sense to remove constraints from the data for performance, maybe. But since the overwhelming majority of databases are read oriented, you're sacrificing performance, not enhancing it. 

And none of this mentions the fact that data integrity is better handled at the database since you only have to create it once where as if you do all the work in code, you may have to do it multiple times for multiple apps (unless you design your data access layer carefully and require every app access the db to go through that same layer).

If you're using a relational database system, I say, why not really use it. If you don't need relational data, go with Hadoop or something else.