Check your web.config file for compilation mode. While you're developing, you should run your site in debug mode. If you're running in release mode, every aspx and ascx will be compiled into temporary assemblies, not only the pages and controls you hit. Also, set batch to false.
<compilation debug="true" batch="false" ...>
Of course, these are not recommended for production environments, which will be slowed down by these changes.
Also, check this forum thread, where you can find some interesting tips, like resetting the permissions of the ASP.NET temporary files folder:
I had the same problem and made all of
the adjustments as mentioned in this
forum but it didn't help. I began to
suspect that the slow loading time of
the dlls had to do with security
settings. It appears that access to
the dlls goes through security
checking that is part of the operating
system and not part of VIsual Studio.
The dlls are located in a directory
for your application under:
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\
In order to speed up the security
checking, you need to remove all of
the security settings on the root
folder of your application in the
above mentioned directory, or on the
directory "Temporary ASP.NET Files" if
you want to have it apply to all web
applications you develop.
You right mouse click the folder and
select Properties -> Security
Remove all users and groups. Some will
not be allowed to be removed because
they inherit their permissions from
their parent folder. You must first
remove the inheritance by clicking on
the Advanced button and then uncheck
the "Inherit from parent..." checkbox.
Then remove the user or group. There
must not be any users or groups where
the checkboxes for their permissions
are greyed out. If they are greyed
out, it means that you didn't remove
the inheritance from their parent.
After you have removed all users and
groups from the folder, you then need
to add the following users and groups:
Administrators Users ASPNET Network
Network Service Local Service
Give each of these accounts full
permission. Click on the Advanced
button again and check off the
"Replace permission entries on all