We were using Ajax Control Toolkit for a lot of UI controls. But it has some serious performance issue. After comparing several client side javascript libraries we ended up with using JQuery. The core JQuery library is just about 20K compressed. It's significantly smaller than Ajax Control Toolkit. With the new page editor we are able to cut the first page load time into half. On average US broadband you should expect less than 2 seconds load time on the first page hit. 

We also improved the page editor UI in a great deal. You can drag and drop any blocks. All the block configurations are happened in the same page. Below is a screenshot of the new page editor.