First call of dbcontext freezes an app after each app reboot

We have a test WPF application (.NET Core 3.0) with only one button in center of the screen. The button have a click event and a method for its processing:

private async void CreateUsers(object sender, RoutedEventArgs e)
    {
        using (var context = new AsyncDbContext())
        {
            await context.Users.AddRangeAsync(new List<User>
                {
                     new User{Name="1"},
                     new User{Name="2"},
                     new User{Name="3"},
                     new User{Name="4"},
                     new User{Name="5"},
                     new User{Name="6"},
                     new User{Name="7"},
                     new User{Name="8"},
                });
            await context.SaveChangesAsync();
            MessageBox.Show("Done!");
        }
    }

This method is async, so we expecting, that UI will not freeze after button click. But, if we start the app and click button, the app freezes on 1-2 seconds. If we will make another clicks it works as expected without freezes.

After it we reboot our app and the situation come again.

1. Database was created
2. .Net framework work exactly the same

3. DbContext:

   public class AsyncDbContext:DbContext
   {
       public DbSet<User> Users { get; set; }
   
       protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
       {
           optionsBuilder.UseSqlServer("Server=ServerName;Database=AsyncDb3; Trusted_connection=true");
       }
   }
   

4. await Task.Delay(5000); work normally

5. Wrapping these actions in await Task.Run(...) work normally

Why does dbcontext freeze WPF app each first click after reboot?