PageSpeed: Avoid document.write (deprecated)
Using document.write() to fetch external resources, especially early in the document, can significantly increase the time it takes to display a web page.
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Details from Google
first.js contains the following content:
The download of first.js and second.js will be serialized in all browsers. Using one of the recommended techniques described in the sidebar can reduce blocking and serialization of these resources, which in turn reduces the time it takes to display the page.