HTML Pages Speed Optimisation

We require the html pages on our website optimised to score very highly for both mobile and Desktop on GTmetrix, Google pagespeed insights and pingdom, expample here: https://www.scotsconnection.com/clan_crests/abercrombie.htm I've done some work on these by combining images with css sprites and have rank of high 90's/low 90's GTmetrix , but I lack the coding skills to get the page scores up by deferring javacript etc