@suhaila I’ve similar lag problems with cosmos:browserify. You might be encountering the same issue: Sometimes cosmos:browserify freezes (loads forever?). · Issue #17 · elidoran/cosmos-browserify · GitHub
Why are you using cosmos:browserify? Now that Meteor 1.3 has introduced ES2015 modules, you can use that instead. Thread with links on how to use Meteor 1.3 Beta: Meteor 1.3 *early beta* now available