You can use the StopWatch class or just set a DateTime field with
DateTime.Now at the very beginning of page processing (you pick the event you
want to start with) and then say, in Page_Unload, you would set a new
DateTime field to DateTime.Now. Take the timespan difference by subtracting
the first from the last, and get the TotalMilliseconds.
Of course, reading between the lines, this is only going to tell you
something you already know: your page is taking a long time to display data.
Better to investigate why this may be happening and work to create a more
efficient way to get what you want.
Peter