Here's an interesting observation, since you mention nHibernate being slow as a consequence of LINQ being slow. If you're doing LINQ to SQL (or the nHibernate equivalent), then your LINQ code translates to an EXISTS query on the SQL server where as your loop code must first fetch all rows, then iterate over them. Now, you could easily write such a test so that the loop code reads all the data once (single DB lookup) for all 10K runs but the LINQ code actually performs 10K SQL queries. That's probably going to show a big speed advantage for the loop-version which doesn't exist in reality. In reality a single EXISTS query is going to outperform the table scan and loop every time -- even if you don't have an index on the column being queried (which you probably would if this query is done very often).
I'm not saying that it is the case with your test -- we don't have enough code to see -- but it could be. It could also be that there really is a performance difference with LINQ to Objects, but that may not translate to LINQ to SQL at all. You need to know what you're measuring and how applicable it is to your real world needs.
