How to implement multithreading in order to make my winform app faster ?

First, if you want to delay any of this computation until after the form is displayed, you should trigger that computation in the handler for the Shown event instead of the Load event.
(See: Form.Shown Event (System.Windows.Forms)[^])

Next, you didn't answer my earlier question about whether or not the results of the Global and ChildAccess methods are shared by the sr_created and sr_solved methods, or if they need to be called independently by sr_created and sr_solved.
I'm going to assume the results are shared and that sr_created and sr_solved should not be executed until Global and ChildAccess have completed.

Also, you haven't said if the UI should block while all of this computation is running (I'll assume no) and if there is something that should happen when the computation of sr_created and sr_solved have completed (I'll assume yes).

So try something like this:
(Note: untested! Coded "off the top of my head" (with some research).)

private void overview_Shown(object sender, EventArgs e)
{
  ComputationChain().ContinueWith(DoAfterComputationCompleted);
}

private Task ComputationChain()
{
  return PreworkComputation().ContinueWith(_ =>
      Task.WaitAll(
        Task.Run((Action)SrCreated),
        Task.Run((Action)SrSolved)));
}

// if Global and ChildAccess can be parallelized:
private Task PreworkComputation()
{
  return Task.WhenAll(
    Task.Run((Action)Global),
    Task.Run((Action)ChildAccess));
}
// if Global and ChildAccess must be run in sequence:
private Task PreworkComputation()
{
  return Task.Run(() => {
      Global();
      ChildAccess();
    });
}
 
//method Sr_created
void SrCreated()
{
  // Global(); //call Global method
  // ChildAccess();//call d_child method
  //all the code of Sr_created()
}
 
//method sr_solved
void SrSolved()
{
  // Global(); //call Global method
  // ChildAccess();//call d_child method
  //all the code of Sr_solved()
}
 
//method Global
void Global()
{
  //all the code of Global()
}
 
//method d_child

void ChildAccess()
{
  //all the code of d_child
}

private void DoAfterComputationCompleted(Task _)
{
  // ignore the Task parameter _
  // "this" should be the instance of the Form
  this.Invoke(() => {
      // the .Invoke(...) gets this to be done on the UI thread
      // here you perform the UI work after the computation has completed
    });
}