GridView pagination works faster in Modal Popup than in page

I have a GridView on a page. The GridView is bound to a DataTable and shows 10 records per page. There are about 40 pages.

I have another GridView, which is shown inside a Modal Popup (AJAX Control Toolkit).

I noticed that when I navigate across pages in a page, the navigation is slow. Whereas inside Modal Popup, pagination is very smooth and fast.

Why is it so?