>>Viv,
>>
>>After I switched to List and Linq, the performance is 10 times slower than it was :( The procedure used to execute in ~800 ms. Now it executes in 8000 ms.
>
>As John says, must be something else going on. Linq should be faster then a coded iteration....

Right, I bet the problem is here with the CAST (that's what I was getting yesterday night)

 columns = (from DataRow r in reader.GetSchemaTable().AsEnumerable()
                                                  select new ColumnSchema
                                                      {
                                                          ColumnName = (String) r["ColumnName"],
                                                          ColumnSize = (Int32) r["ColumnSize"],
                                                          DbType = (SqlDbType) r["ProviderType"],
                                                          DataType = reader.GetFieldType((int)r["ColumnOrdinal"]),
                                                          Precision = (Int16) r["NumericPrecision"],
                                                          Scale = (Int16) r["NumericScale"],
                                                          IsIdentity = (Boolean) r["IsIdentity"]
                                                      }).ToList();

I am going to debug it in a few minutes and see what needs to be adjusted.