There is really no way to make a progress bar open "while a page loads" since the progress bar won't load until the page does.
The progress bar is generally used for while a page submits, or while the next page loads.
You could create a loading page that the user goes to first that has the progress bar, then have that page redirect to the next page that loads slowly, then they will see the progress bar page until the next page loads.
I could probably also help you fix the page so it doesn't take so long to load, which is probably the better option. Pages really shouldn't take that long to load, so I'd suggest trying to solve that problem instead of putting a band-aid on it with a progress bar.