The very first time that the page is load, then the asp.net compile a lot of pages, almost every one found on the same dir, including modules, and dlls found on bin.
To speed up (your development) try use this option on web.config (only on your development computer).
<compilation batch="false" ... >
Using batch="false" asp.net compiles only one page, the one you are on it, so you get a big time the first time for the dlls, but after that, is a lot less if you have 200 pages, and you only change one, and you develop/debug only one.
Also if you can, try do not use App_Code directory, and place all your code inside a dll.
Look more about compilation options on web.config to make it even run faster, change for example the temporary directory to a faster different disk than the C:.
Second trick
I will like to say one more trick that I found here on stackoverflow.
<compilation optimizeCompilations="true">
You only need to have install an ms patch http://support.microsoft.com/kb/961884
I have test it and the results are very good.
.net 4 and Visual studio 2010
The same think stands, but you do not need to run the patch.
Web Farm
I have notice that on web farm the first time that application is run, if you have many work on global.asax on application start, then I recoment to use Mutex and let only one pool make the compile of the pages ! Or else there are possibilities for conflicts and huge delay to start the application - especial if you use mutex on other part of the program.
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