Why is LINQ JOIN so much faster than linking with WHERE?

I've recently upgraded to VS 2010 and am playing around with LINQ to Dataset. I have a strong typed dataset for Authorization that is in HttpCache of an ASP.NET WebApplication.

So i wanted to know what actually is the fastest way to check if a user is authorized to do something. Here is my datamodel and some other informations if somebody is interested.

I have checked 3 ways:

1. direct database
2. LINQ query with Where conditions as "Join" - Syntax
3. LINQ query with Join - Syntax

These are the results with 1000 calls on each function:

1.Iteration:

1. 4,2841519 sec.
2. 115,7796925 sec.
3. 2,024749 sec.

2.Iteration:

1. 3,1954857 sec.
2. 84,97047 sec.
3. 1,5783397 sec.

3.Iteration:

1. 2,7922143 sec.
2. 97,8713267 sec.
3. 1,8432163 sec.

Average:

1. Database: 3,4239506333 sec.
2. Where: 99,5404964 sec.
3. Join: 1,815435 sec.

Why is the Join-version so much faster than the where-syntax which makes it useless although as a LINQ newbie it seems to be the most legible. Or have i missed something in my queries?

Here are the LINQ queries, i skip the database:

Where:

Public Function hasAccessDS_Where(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule, _
                roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule, _
                role In Authorization.dsAuth.aspnet_Roles, _
                userRole In Authorization.dsAuth.aspnet_Use