LINQ Join vs SelectMany:what is faster?

Customer[] customers = Customers.ToArray();
Purchase[] purchases = Purchases.ToArray();

var slowQuery =
        from c in customers
        from p in purchases where c.ID == p.CustomerID
        select c.Name + " bought a " + p.Description;

var fastQuery =
        from c in customers
        join p in purchases on c.ID equals p.CustomerID
        select c.Name + " bought a " + p.Description;

slowQuery.Dump ("Slow local query with SelectMany");
fastQuery.Dump ("Fast local query with Join");

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