[smarks159]

The reason polymer is slow on firefox and not on chrome is because firefox does not support html imports yet, so polymer uses a polyfill that relies on asynchronously loading the html which causes a delay in the initial load. Support for html imports is not implemented in safari either.[1]

[1] http://caniuse.com/#feat=imports

[spankalee]

The HTML imports polyfill shouldn't be much slower than native, since native imports are also asynchronous, but block execution of subsequent scripts. Any (small) slowdown would be from the browsers pre-scanner not knowing to optimistically load imports. This can be avoided completely by using the `vulcanize` tool which bundles all imports into one file.