Hi Herdy Handoko,
Thank you for using our services.
The issue is comming by the fact that you do not use the script manager controls on your master pages to handle script combining so that you minimize the amount of the scripts loaded. Since we heavily use the RadControls for ASP.NET AJAX within Sitefinity controls the browsers make script loading requests for each control that they have to render, instead of loading the scripts at once. This is why the page size gets a little bit bulky.
All AJAX controls need a script manager on the master page to work. Now you can take a look at the following documentation page
on how to utilize the RadScriptManager control on your master page in order to load less scripts on page loads.
the Telerik team