I haven't used Sencha Architect but used an earlier version of Sencha for a phonegap app, maybe some of same considerations apply.
You can use the Chrome Audits tab to find unused css and remove them. It also give a lot more suggestions on how to improve your code.
Few more suggestions: ref
- Use far-future cache expiration headers. This will prevent the browser from having to send a conditional GET request.
- Try to limit HTML pages to 25.6KB or less if you want them to be cached, since the previous tests showed that this limit—imposed by iOS 3.2 on the iPad—was the lowest HTML resource limit of the devices tested.
- Keep CSS and JS components under 1MB. Of course, 1MB is enormous and your components should be much smaller than this, but don’t bother splitting a component into separate requests for the sake of cacheability unless its size approaches 1MB.
- Consider using the HTML5 application cache if it’s important that your components persist in the cache for a long time, or across power cycles.
Also don't forget the basic stuff, such as turning images to inline svg to cut down on requests made & moving the Javascript includes to the bottom of the page.
If you are doing animations, use css 3d transforms. This makes use of the GPU rather than the 2d transforms. eg: translate3d()
Hope this helps, good luck!