WCF/WF - Speed up linq query ?

Hi may U try help me to optimize this query, because it work very slowly:

public List<SzukajPremium> SzukajPremium(string kod, string rok, string numer)
        {
            DataContext db = new DataContext();                                
            var result = from z in db.Zeszyts
                        join p112 in db.ZeszytP112s on z.IdZeszyt equals p112.IdZeszyt
                        join dok in db.Doks on z.IdZeszyt equals dok.IdZeszyt
                        select new SzukajPremium { P182 = dok.P180, Tytul = dok.Tytul, IDPUB = string.Format("{0} [dodatek: {1}] - {2}.{3}", z.P101, z.P133,dok.P180.Substring(4,12),dok.P180.Substring(16,7).Replace("0","")), KodDodatku=z.Nazwa.Substring(13,3), Rok=z.P110, Numer=p112.P112 };
             if (!string.IsNullOrEmpty(kod))
                result = result.Where(k => k.KodDodatku == kod);
            if (!string.IsNullOrEmpty(rok))
                result = result.Where(r => r.Rok == rok);
            if (!string.IsNullOrEmpty(numer))
                result = result.Where(n => n.Numer == numer);            
            return result.ToList();
        }

There is no doubt that tehre are some drawbacks to Linq to Sql. One of them is that the Sql statement is built dynamically so it is needed to be parsed and compiled each time you run it. Fortunately .Net 3.5 has a solution for this problem. System.Data.Linq namespace includes a class named CompiledQuery which is responsible for caching the compiled version of a Linq to Sql query. This class has a static method called Compile which takes a Func<T,S,R> delegate. In this signature, T is the type of a DataContext (i.e. HRMDataContext) , S is the type of a predicate to filter the query and R is the type of returned result. Needless to say that it must be IQueryable<T>.

In this article we will see how to pre-compile a query, its limitations and how it really improves the speed of a Linq query.
More details here
http://aspguy.wordpress.com/2008/08/15/speed-up-linq-to-sql-with-compiled-linq-queries/
http://weblogs.asp.net/wesleybakker/archive/2009/07/30/how-to-make-a-linq-to-sql-query-twice-as-fast.aspx

So that should look like this?

public static Func<DataContext , SearchCriteria, IQueryble<SzukajPremium>> FilteredResult =
System.Data.Linq.CompiledQuery.Compile(
(DataContext dc , SearchCriteria criteria ) =>
var result = from z in db.Zeszyts
                        join p112 in db.ZeszytP112s on z.IdZeszyt equals p112.IdZeszyt
                        join dok in db.Doks on z.IdZeszyt equals dok.IdZeszyt
                        select new SzukajPremium { P182 = dok.P180, Tytul = dok.Tytul, IDPUB = string.Format("{0} [dodatek: {1}] - {2}.{3}", z.P101, z.P133,dok.P180.Substring(4,12),dok.P180.Substring(16,7).Replace("0","")), KodDodatku=z.Nazwa.Substring(13,3), Rok=z.P110, Numer=p112.P112 };
             if (!string.IsNullOrEmpty(kod))
                result = result.Where(k => k.KodDodatku == kod);
            if (!string.IsNullOrEmpty(rok))
                result = result.Where(r => r.Rok == rok);
            if (!string.IsNullOrEmpty(numer))
                result = result.Where(n => n.Numer == numer);            
            return result.ToList();

);