There may be many reasons behind the slowing down of the Website.
You have to research all the modules of Website carefully and check which components are exactly making it slow.
You can see the
POST/GET
requests inside
FireBug
"
Net" tab (in Firefox), which are sent to your application, while the page is loaded. You can see the time each request takes there only.
After that you will be a bit particular of the functions slowing down the site.
For example, when you click on Sign-in button, there will be request in the "
Net" tab, which will show the actual time taken to complete that task. If that takes more time than you expect, then you have to check that Sign-in function by debugging and find the exact cause.
And yes, as you mentioned that it slows down in evening, so you need to detect the number of users coming to your site at that time and how to control that. There is also a setting in IIS to enable maximum Limits of connection. That is at...
IIS-> Website-> Advanced Settings-> Connection Limits
There are many Performance Improving Techniques, which you can go for.
Google [
^]will suggest you many articles in this topic.
Now, you just need to follow these as far as you can and make it better performed.