ExtJS 4 is much faster than ExtJS 3. Guys from sencha solved a lot of rendering problems and improved performance greatly.
In the sample link just read the deployment section. The performance there is normal issue for development as in 4th version was introduced ext loader and it is much better for debugging and development using concrete short files of the framework instead of loading big files (more than 100k lines) and you always knows where the problems are.
In production you need to use Sencha SDK which will generate yours application initial required js plus extjs dependencies. It is much better than it was in 3rd version where you need to load all the framework.
You can run Sencha CMD, a ruby gem which dynamically creates your ExtJS 4 css file, allowing you to cut the full css file (~ 500kb !) down to something less ridiculous.
It may seems to be late reply, but the truth is ExtJS 4.x is much fast than ExtJS 3.x version, more over it made front-end developer to create their application with a MVC pattern approach to organize their coding structure much more clearer an maintainable.
ExtJS 4.x solve rendering problems an improved in performance over browser compatibility also.
To answer your question, it may be many reason as stated below.
Check with the developer tool, which part of js file taking more time to load as compare to extjs 3.x
you may be using more inner components(adding more items to parents and child component having more sub-components,etc).
Even you can use the newly-introduced Sencha SDK Tools (download here) makes deployment of any Ext JS 4 application easier than ever. The tools allow you to generate a manifest of all JavaScript dependencies in the form of a JSB3 (JSBuilder file format) file, and create a custom build containing only the code that your application needs.
Note: Its all depends on, how you design your application (Architect).
ExtJS 4.x is contain lot of bugs in comparison ExtJS 3.x. I am using both in my different project. for example 4.0.7 contain file upload issue till now so i want to tells my opinion extjs 3.x is more stable release of extjs frame work.
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ExtJS 4.0 有性能问题，
ExtJS 4.0.2 是您需要的修复大多数性能问题的版本，但它仍然很慢...
Google for ExtJS 4 性能
ExtJS 4.0 has performance issues,
ExtJS 4.0.2 is the release you need that fixes most performance issues, it is still slow though...
Google for ExtJS 4 performance
您是否忘记关闭动态资源加载？请注意，ext*-all.js 将显着加快页面加载速度。
Did you forget to turn off dynamic resource loading? Note that ext*-all.js will speed up your page loading dramatically.
ExtJS 4 比 ExtJS 3 快得多。sencha 的人解决了很多渲染问题并大大提高了性能。
在示例链接中，只需阅读部署部分。开发中的性能问题是正常的，因为在第 4 版中引入了 ext loader，使用框架的具体短文件而不是加载大文件（超过 100k 行）进行调试和开发要好得多，而且您始终知道问题出在哪里是。
在生产中，您需要使用 Sencha SDK，它将生成您的应用程序初始所需的 js 以及 extjs 依赖项。它比第三个版本要好得多，第三个版本需要加载所有框架。
ExtJS 4 is much faster than ExtJS 3. Guys from sencha solved a lot of rendering problems and improved performance greatly.
In the sample link just read the deployment section. The performance there is normal issue for development as in 4th version was introduced ext loader and it is much better for debugging and development using concrete short files of the framework instead of loading big files (more than 100k lines) and you always knows where the problems are.
In production you need to use Sencha SDK which will generate yours application initial required js plus extjs dependencies. It is much better than it was in 3rd version where you need to load all the framework.
您可以运行 Sencha CMD，这是一个 ruby gem，它可以动态创建您的 ExtJS 4 css 文件，允许您将完整的 css 文件（~ 500kb！）削减到不那么荒谬的程度。
我在四月份在这里概述了这个过程 https://docs.google.com/document/d/1lGfbEoktJNhIpZneeJ2kkjNKOaSYpOse3mnSA5mGEds/edit?usp=sharing
You can run Sencha CMD, a ruby gem which dynamically creates your ExtJS 4 css file, allowing you to cut the full css file (~ 500kb !) down to something less ridiculous.
I have outlined this process in April here https://docs.google.com/document/d/1lGfbEoktJNhIpZneeJ2kkjNKOaSYpOse3mnSA5mGEds/edit?usp=sharing
Extjs 4.1 解决了许多与布局相关的问题。如果您有嵌套面板，与 Ext 3 相比，渲染效果更好。
Extjs 4.1 is resolved many issues related to the layouts. If you have nested panels, does better rendering when compared to the Ext 3.
注意：这完全取决于您如何设计应用程序（架构师）。
谢谢
Note: Its all depends on, how you design your application (Architect).
Thanks
与 ExtJS 3.x 相比，ExtJS 4.x 包含很多错误。我在不同的项目中使用两者。例如4.0.7到目前为止包含文件上传问题所以我想告诉我的意见
extjs 3.x 是 extjs 框架的更稳定版本。
ExtJS 4.x is contain lot of bugs in comparison ExtJS 3.x. I am using both in my different project. for example 4.0.7 contain file upload issue till now so i want to tells my opinion
extjs 3.x is more stable release of extjs frame work.
使用Extjs4.0或更高版本的MVC功能
将商店视图模型控制器放在不同的文件夹中
并使用控制器调用它们。
Use MVC feature of Extjs4.0 or later version
put store views model controller in different folder
and call them using controller.