Why might OpenFileDialog::ShowDialog() take ~5 seconds to return after clicking the 'OK' button?

I have the following code to prompt a user for a file to open:

System::Windows::Forms::OpenFileDialog fd;
fd.Filter = "All files (*.*)|*.*";
fd.RestoreDirectory = true;
if (fd.ShowDialog() == System::Windows::Forms::DialogResult::OK)
{
    m_lblProgress->Text = "Opening file";                
}

For some reason, after I click the OK button on the dialog, it takes ~5 seconds before the text on the GUI is updated and it's unresponsive until that happens.

Why might that be and how can I avoid it?

Edit - The plot thickens..

This delay only appears when the OpenFileDialog is launched from a System::Windows::Forms::ToolStripMenuItem::Click event.

Changing a label's text instead of launching the OpenFileDialog doesn't incur any delay at all so what's special about closing the OpenFileDialog window?