Left-Outer-Join to same table with Linq/EntityFramework - entity-framework

Question

I need to create a left-outer-join to the same table:
SELECT Top(20) s.Id as ParentId, CASE WHEN cr.Id IS NOT NULL THEN 'False' ELSE 'True' END AS HasChangeRequest
FROM [Platos.PRM.Dev].[dbo].[Section] s
left outer join [Section] cr on cr.ParentSectionId = s.Id
For this I wrote the following Linq statement:
var query = from section in this.prmDbContext.Sections
join changeRequest in this.prmDbContext.Sections.Where(x => x.ParentSection != null && !x.IsInactive) on section.Id equals changeRequest.ParentSection.Id into changeRequestJoin
from changeRequest in changeRequestJoin.DefaultIfEmpty()
select new
{
Id = section.Id,
HasChangeRequest = changeRequest != null
};
var result = query.Take(20).ToList();
The linq statement works and deliver the correct result, but is very slow.
SQL statement: some millisconds - LINQ statement: 5 seconds
Could anyone tell me, why the performance with this linq statement is bad?
Thanks!
Update:
After adding a navigation property (ChangeRequests) I update the query to the following linq statement:
var query = from section in this.prmDbContext.Sections
select new
{
Id = section.Id,
HasChangeRequest = section.ChangeRequests.Any()
};
var result = query.Take(20).ToList();
Now the EntityFramework creates the following sql query:
SELECT
[Limit1].[C1] AS [C1],
[Limit1].[Id] AS [Id],
[Limit1].[C2] AS [C2]
FROM ( SELECT TOP (20)
[Extent1].[Id] AS [Id],
1 AS [C1],
CASE WHEN ( EXISTS (SELECT
1 AS [C1]
FROM [dbo].[Section] AS [Extent2]
WHERE [Extent1].[Id] = [Extent2].[ParentSectionId]
)) THEN cast(1 as bit) ELSE cast(0 as bit) END AS [C2]
FROM [dbo].[Section] AS [Extent1]
) AS [Limit1]
Now the query is faster, but still slower than the left-outer-join from the native sql query. So is it possible, to optimize the linq statement?