I plan to develop mobile app. I came across ionic and polymer framework. Which one is best for mobile UI and performance?
First, before choosing any of these frameworks you need to know.
Ionic is based on Angular.js which is a very powerful framework for building SPA and Ionic uses Angular's Directives to make custom HTML tags.
But whereas Polymer is just a library that makes extensive use of Web Components(New set of standards developed by the W3C and is not implemented by all browsers and previous releases of Android).
Up until now Polymer has been only an experiment so it's not production ready to be used for deployment, but this could change soon after Polymer 0.8 is released.
After knowing some info about these frameworks, I would go for Ionic cause I know it is widely used by developers and is production ready, on the other hand if you wish to wait for Polymer 0.8 to be released(expected by March-2015,)it has it's own set of features that are worth waiting for.
Unlike other frameworks, Polymer and its polyfills are intended to work in the latest version of “evergreen” browsers. This means that your favorite browser might not be supported. https://www.polymer-project.org/0.5/resources/compatibility.html
So unless you can afford to sacrifice compatibility, go with the safer choice.