How can I optimize this LINQ statement for speed?

The following LINQ statement finds items with duplicate values for a particular property, groups them by count then does another grouping to get a boolean back.

I'm just curious how this can be improved, it seems a bit wasteful and as this is part of a object model validation library I'd quite like to get it as speedy as I can.

Speed of execution is a priority, but any other suggestions are welcome.

var grouped = from g2 in
                 (from i in item.ParentList
        开发者_JS百科          where _filter(i)
                  group i by propGetter(i) into g
                  select new { Count = g.Count(), Items = g })
              group g2 by g2.Count == 1 into g3
              select new { IsUnique = g3.Key, Items = g3 };

foreach (var g in grouped)
{
    foreach (var grp in g.Items)
    {
        foreach (var itm in grp.Items)
        {
            if (g.IsUnique == false)
                itm.AddPropertyError(_propertyName, (int)Validations.Unique, _message);
            else
                itm.RemovePropertyError(_propertyName, (int)Validations.Unique);
        }
    }
}

As Oskar said, you probably don't want to optimize the query for speed unless profiling shows that it's causing a problem. Premature optimization is the root of all evil. If you want to optimize the query for readability, here is one way to simplify the second part:

var items = grouped
    .SelectMany(group => group.Items)
    .SelectMany(group => group.Items)

foreach (var item in items)
{
    ...
}

Edit regarding your comment: Ah, I hadn't noticed that you were referencing g.IsUnique inside the innermost loop. Here is one way to resolve that problem without 3 levels of indentation, but it may not be the best way:

var uniqueItems = grouped
    .Where(group => group.IsUnique)
    .SelectMany(group => group.Items)
    .SelectMany(group => group.Items)

var nonUniqueItems = grouped
    .Where(group => !group.IsUnique)
    .SelectMany(group => group.Items)
    .SelectMany(group => group.Items)

foreach (var item in uniqueItems)
{
    ...
}

foreach (var item in nonUniqueItems)
{
    ...
}