While reading through the excellent http://blogs.msdn.com/ I noticed this post by Todd Carter talking abount ASP.NET and SQL Server Performance, in it he talks about compiler locks slowing down stored proceedure execution. Aparently this occurs when

1. The user that runs the stored procedure is not the owner of the procedure AND
2. The stored procedure name is not fully qualified with the object owner’s name.

You can read the full article here.