>> Sometimes and in some clients the popup form displays blank, see image.
If the form remains blank then check the Firebug Console Log. Typically it means you've broken the form with some non-xhtml text.
>> I´ve seen that, in a slow machine or a slow connection I see the blank form first and then display the complete form.
This is by design. It's desirable to give the user immediate feedback when they press the button, and then populate the contents of the form when it arrives.
cheers
Bruce