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There are 2 recommended snippets for including Boomerang JS performance tracking snippet. Both of them do not block other resources but the one including Boomerang JS via Iframe also do not block the onload browser event. Link to official documentation.

1) Via Iframe

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<script>
(function(){
    // Boomerang Loader Snippet version 10
    if (window.BOOMR && (window.BOOMR.version || window.BOOMR.snippetExecuted)) {
        return;
    }

    window.BOOMR = window.BOOMR || {};
    window.BOOMR.snippetExecuted = true;

    var dom, doc, where, iframe = document.createElement("iframe"), win = window;

    function boomerangSaveLoadTime(e) {
        win.BOOMR_onload = (e && e.timeStamp) || new Date().getTime();
    }

    if (win.addEventListener) {
        win.addEventListener("load", boomerangSaveLoadTime, false);
    } else if (win.attachEvent) {
        win.attachEvent("onload", boomerangSaveLoadTime);
    }

    iframe.src = "javascript:void(0)";
    iframe.title = "";
    iframe.role = "presentation";
    (iframe.frameElement || iframe).style.cssText = "width:0;height:0;border:0;display:none;";
    where = document.getElementsByTagName("script")[0];
    where.parentNode.insertBefore(iframe, where);

    try {
        doc = iframe.contentWindow.document;
    } catch (e) {
        dom = document.domain;
        iframe.src = "javascript:var d=document.open();d.domain='" + dom + "';void(0);";
        doc = iframe.contentWindow.document;
    }

    doc.open()._l = function() {
        var js = this.createElement("script");
        if (dom) {
            this.domain = dom;
        }
        js.id = "boomr-if-as";
        js.src = 'http://your-cdn.host.com/path/to/boomerang-<version>.js';
        BOOMR_lstart = new Date().getTime();
        this.body.appendChild(js);
    };
    doc.write('<bo' + 'dy onload="document._l();">');
    doc.close();
})();
</script>

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2) Adding to the main document

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<script>
(function(d, s) {
var js = d.createElement(s),
    sc = d.getElementsByTagName(s)[0];

    js.src="http://your-cdn.host.com/path/to/boomerang-<version>.js";
    sc.parentNode.insertBefore(js, sc);
}(document, "script"));
</script>

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