How to quickly load related objects

I would like to load Test

by id including all related TestRuns

and everything Measurements

using DbContext / EntityFramework from MySql database.

This is the database schema:

What I have tried so far:

public class TestRepository : Repository<Test>, ITestRepository
{
    public IQueryable<Test> GetTestComplete(int id)
    {
      return DbSet.Where(t => t.Id == id)
                  .Include(t => t.TestRuns.Select(tr => tr.Measurements));
    }
}

      

        
        
        
      

    

Unfortunately, it takes a very long time (about one minute for 1 test / 1 Testrun / 15000 Measurements). I tried to understand the generated SQL using Query Profiler but couldn't understand the huge monster SQL statement.

Can you think of a better (i.e. faster) way to load data using DbContext?

Update

Another try, also resulting in looong load times:

public Test GetTestComplete(int id)
{
    Test test = DbSet.Find(id);
    DbContext.Entry(test).Collection(t => t.TestRuns).Load();
    foreach (var testRun in test.TestRuns)
    {
        // next line takes a lot of time!
        DbContext.Entry(testRun).Collection(tr=>tr.Measurements).Load(); 
    }
    return test;
}

      

        
        
        
      

    

Loading measurements takes 84% ​​of the time:

This is the appropriate sql statement for extracting dimensions:

SELECT 
Extent1.id,
Extent1.test_run_id,
Extent1.rss_dbm
FROM measurement AS Extent1
WHERE Extent1.test_run_id = :EntityKeyValue1

      

        
        
        
      

    

I copied each of the resulting SQL queries (from the 3 queries DbContext

/ DbSet

) from the Query Profiler in MySqlWorkbench and each one is very fast. Now I am even more confused ...

Update 2

I've isolated the function GetTestComplete

(see above) in a unit / performance test and it still takes a long time. The output from the query profiler shows that the individual sql commands are very fast, although the whole test takes about 5 seconds. Confusion grows ...