The application pool identity should be set to the user that has permissions over the App_Data folder.
Can you also verify that the server has fulfilled all requirements by Sitefinity 4 and the application pool is running using .NET 4 framework and Integrated Pipeline mode?
Please verify that all your system assemblies ( those referenced in your project ) are for the correct .NET version that is installed on your server.
We have browsed your beta website, but the pages were loading quite fast. However most of the loading time was absorbed by some custom .js scripts you have added to the template / or the pages.
Also you may want to verify that Output cache is enabled:
The setting can be found under administration -> settings -> advanced -> system -> output cache.
@Siddesh: please try the above suggestions as well.
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