Optimize page

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According to several analyzers, current page load time could be optimized:

• https://developers.google.com/speed/pagespeed/insights/
• http://nibbler.silktide.com/
• http://www.websiteoptimization.com/services/analyze/
• https://www.giftofspeed.com/
• https://gtmetrix.com/
• https://www.pagespeedguide.com/
• https://varvy.com/pagespeed/
• https://try.powermapper.com/Demo/SortSite
• https://www.computerhope.com/issues/ch001274.htm
• https://mxtoolbox.com/domain/
• https://website.grader.com/
• http://www.webpagetest.org/
• https://goingup.com/analyzer
• https://tools.pingdom.com/
• More services with may be added later

This issue is generic, concrete issues will be linked here.