The problem is that in our original query we’re not getting data from the LinkedPosts entity, just data from Posts and PostTags. Entity Framework knows that it doesn’t have the data for the LinkPosts entity, so it very kindly gets the data from the database for each row in the query results.
Obviously, making multiple calls to the database instead of one call for the same data is slower. This is a perfect example of RBAR (row by agonizing row) processing.
Read the comments for more answers on top of Richie’s. My answer (only 70% tongue in cheek)? Functional programming languages don’t require ORMs.