You just need two simple opencart extension to achieve a perfect performance score.
Optimized images and CDN
what stock opencart lacks according to https://gtmetrix.com are:
Optimized images. wich means the stock images are not to scale, and they are not compressed or optimized
Utilizing a CDN network to offload and download resources in parallel, spread the workload and distribute the files worldwide to servers located physically closer to the client.
to improve image optimization and deliver them via a CDN network. we need the Opencart cloudinary CDN extension for opencart
Fewer http requests