Reduce or improve Linq query with nested from-where clauses
After a lot of setup, I arrived at this:

var extensionMethodQuery =
    from container in containers
    let items = container.GetItems()
    select items.GetItemDerivedTagItems
        (items.SelectInterestingItemTags()).FirstOrDefault();


Your original query looks pretty optimal, but it took a while to see why.  I broke it down into its component parts, and concluded that I'd concentrate on making it more reusable and readable.  Personally, I find lambda joins too verbose, so created two extension methods instead:

public static class Filters
{
    public static IEnumerable&lt;ItemTag&gt; SelectInterestingItemTags
        (this IEnumerable&lt;Item&gt; items)
    {
        return
            from item in items
            let tag = Tag.CreateTag(item)
            where Tag.IsInteresting(tag)
            select new ItemTag(item, tag);
    }

    public static IEnumerable&lt;ItemDerivedTagItem&gt;
        GetItemDerivedTagItems
        (this IEnumerable&lt;Item&gt; items, IEnumerable&lt;ItemTag&gt; itemTags)
    {
        return
            from itemTag in itemTags
            from item in items
            let derivedTag = Tag.CreateDerivedTag(itemTag.tag)
            where Tag.CreateTag(item) == derivedTag
            select new ItemDerivedTagItem(itemTag.item, item);

    }
}


You'll notice I needed to create two extra classes to enable this, ItemTag and ItemDerivedTagItem, that replace the anonymous classes.  Those names are probably awful, but not knowing the context, I did the best I could.  The IsInteresting and CreateDerivedTag methods were made static member methods of their classes, too.  The results were compared successfully via unit tests, against my dummy classes and dummy data, so hopefully you can verify that with yours.

As ever, the proof is in the profiling, and for me at least, the extension method query is 4 times faster.  I can supply the dummy classes and data I used if needed, but as mentioned in the comments, I'd much prefer those supplied next time please!