EF Improve performance of Many-to-Many / .Include .ThenInclude - sql-server

Question

I've got a relatively basic model - Users and Tags. There is a fixed list of Tags. A User can have multiple Tags and a Tag can be used by multiple users.
I had gone with structure below and finding performance issues when returning results.
public class User
{
public string Id {get; set;}
public virtual List<UserTag> UserTags {get; set}
}
public class UserTag
{
public string UserId { get; set; }
public User User { get; set; }
public int TagId { get; set; }
public Tag Tag{ get; set; }
}
public class Tag
{
[Key]
public int TagId { get; set; }
public string Name { get; set; }
public virtual List<UserTag> UserTags { get; set; }
}
I have the following query which is takings a long time (several seconds):
var x = db.Users.Include(u => u.UserTags).ThenInclude(u => u.Trait).ToList<User>();
I have tried writing it as such, which has improved the time, however it is still taking too long:
db.UserTags.Load();
db.Tags.Load();
var x = db.Users.ToList<User>();
Is there any other way to speed this up? Running a query directly in SQL SMS is almost instant (e.g.
select * from Users u left outer join UserTags t on t.UserId = u.Id)
In terms of data rows, it is apx Tags: 100, UserTags:50,000, Users: 5,000

Answer 1

First you can check how EF translates your request to SQL Server - therefore use the "SQL Server Profiler"
Then you could use the genereated query to check if there might be an missing index which speeds up the query
You also can try to write a Join instead of ThenInclude and see how the query then behaves
best regards
Jimmy