I am not an expert on calculating these things, but here are a few points to
keep in mind:

1. When the application uses multiple dll's, the server has more work to do,
therefore slightly increasing the time required
2. The server does cache some pieces of information, sometimes more
depending on the application. Therefore, this cache must be recreated if any
dll's or the web.config file is modified, increasing the time when the
application is run the first time.
3. The speed can also be affected by how much stuff is in viewstate. And
take note that if you have viewstate enabled for a large number of objects,
viewstate can become quite large after a couple postbacks of the same page.
Therefore, it is a good idea to remember to set viewstate to False for
anything reasonable, since it is set to True by default. Many controls such
as Labels or Controls who have their value set in the Load event have no
need for viewstate, when I saw the difference between my pages when I
started remembering to do this it was a big difference in efficiency.
4. Sometimes it is better not to use ASP.NET Controls. For example, if you
have an image that is "just there" and is always the same, just use a plain
HTML img tag for it, this is less code because it does not get declared as a
Control (even if you never reference a Control, Visual Studio .NET
automatically adds the declaration for WebControls, but it does not
automatically add declarations for HtmlControls), therefore creating less
work for the server.
5. Anywhere possible, use subprocedures and/or functions. Even if the
subprocedure/function is only used 2 or 3 times, it is less compiled code
because the compiler only needs to compile it once.

There may be other things you can do, but I am not sure what they might be.
Please don't be offended if I mentioned anything that sounds "common sense",
I remember how much I knew when I first started ASP.NET, and sometimes
simple things aren't simple until someone has told you about them. There are
things that I spent almost a year doing the hard way because I didn't know
there was a better way. Good Luck!