doHeaders method is blocking threads in EPP 5.1

Issue

We have a portlet placed in all the portal pages that includes some tags in the portal

   protected void doHeaders(RenderRequest request, RenderResponse response) {
       super.doHeaders(request, response);
       Element headElem = response.createElement("script");
       headElem.setAttribute("language", "javascript");
       headElem.setAttribute("src", "/javascript/custom.js");
       headElem.setAttribute("type", "text/javascript");
       response.addProperty(MimeResponse.MARKUP_HEAD_ELEMENT, headElem);
       ...
   }

When doing a stress test, we have observed that the first RenderResponse.createElement call blocks the thread. This has a negative effect on performance during our stress test.