I will make your Opencart load in less than 3sec and take average scores to 90+ levels.
The work is done via hand-coding internal server/website files. Caching plugins have a limit to what they can do (usually fail to exploit the maximum speed potential) and generate issues.
Goal is the lowest TTFB and then getting above 90% in accredited online tools (Google Pageinsights, GTMetrix, Webpagetest, etc).
Most of the time I see issues having to do with slow server configuration and website misconfiguration, redundant plugins that slow down performance, non-optimized images, CSS, JS, etc. I fix all errors originating in server/website and optimize TTFB, CSS/JS minification, reducing external calls, adding expires headers, images, etc.
12 yrs experience in Big Data and eCommerce sites (Magento, Prestashop, CS-cart, Woocommerce, OpenCart), WordPress, PHP, CSS, HTML, XML, JSON-LD, Speed optimization, Server Issues, Redirects, Mobile responsive sites, Performance Marketing Analytics, Schema Markup, GSC Errors: Indexation, Sitemaps, Spam Backlinks, Speed, Structured Data, Layout errors, Fast Google Indexation of Backlinks, etc.