How can I make my Dynamic Linq query more efficient - performance

Question

I've written an mvc controller function to handle dynamic queries, it's working great but with some more complicated queries it's really bogging down as much as 15 second response time. I used ObjectQuery.ToTraceString to get the sql and execute it against my database and I'm getting 1-2 second response times, not great but hugely disparate from the time it takes for my server to respond to the same query.
Here is my code:
public class QueryController : Controller
{
//
// GET: /Query/
Entities1 Context = new Entities1();
public JsonResult Query(
string select,
string table = "Product",
string where = "",
string groupBy = ""
)
IQueryable dataTable;
if (table == "Customer") dataTable = Context.Customers;
else if (table == "Product") dataTable = Context.Products;
else if (table == "Purchase") dataTable = Context.Purchase;
else dataTable = Context.Farms;
if (select == null) return null;
string whereClause = where;
string selectClaus = select;
string groupByClaus = groupBy;
IQueryable queryResults = dataTable;
if (where != "") queryResults = queryResults.Where(whereClause);
if (groupByClaus != "") queryResults = queryResults.GroupBy(groupByClaus, "it");
queryResults = queryResults.Select(selectClaus);
JsonResult result = new JsonResult();
result.Data = queryResults;
result.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
return result;
}
}
This code is supposed to handle requests like this:
?table=Purchase
&select=new (Customer.First as Name, Product.Id as ProdId)
&where=Category == 5
A query like the above takes about 700 ms, but if I try something more complicated it slows to a crawl (15 seconds):
?table=Purchase
&select=new (
count(true) as Total,
Key.Product as Product,
Key.CustomerRef as CustomerRef,
count(Price > 475) as BigPurchases,
count(PaidFor == false) as UnPaid,
count((Category != null) and (Comments == null) and Returns == null) as NoFeedback)
&groupBy=new (
Product.ProductName as Product,
CustomerRef as CustomerRef
)
In particular the navigation Property seems to be a problem, removing it speeds up the query considerably (3 sec):
?table=Purchase
&select=new (
count(true) as Total,
Key.Product as Product,
Key.CustomerRef as CustomerRef,
count(Price > 475) as BigPurchases,
count(PaidFor == false) as UnPaid,
count((Category != null) and (Comments == null) and Returns == null) as NoFeedback)
&groupBy=new (
ProductRef as Product,
CustomerRef as CustomerRef
)
The time all seems to get used iterating over the IEnumerable, in the code I provide I pass the data out and let the whatever underlying MVC code do whatever converting it wants, which takes approximately the time mentioned. However if I iterate over it myself, or use the ToList function I get similarly slow response times.
Does anyone have any ideas as to what is causing the long pause getting these entities?
Update
I added indexing to my database which sped things up, but it is still taking 20-30 times as long to execute the query in linq than it does in sql.