.aspx page performance issue

It seems, you are doing some heavy operation (data calling probably) in page_load.
So, until the page_load event completes, there will be a kind of partial display and the page is fully loaded and rendered after the page_load event completes and the html renders.

There are several options but 2 options most widely used to handle this
1. See, how you can make the page load light. Study the as.net page life cycle and see, how you can make the page load method light.
2, Use Ajax to load the data and display in the page.

I will prefer the 2nd one as this is the most convenient and robust as well.
So, the page loads instantly, no database call operation in page_load. Get the data using ajax in client side and display the data in the page.

Hope this gives you some idea to start with.

cheers