For a long time my client struggled with fixing Core Web Vital issues. Someone in his team employed a freelance coder to fix the issues, but it was not successful because the coder wasn't up to scratch.

One of the problems with tackling issues on Shopify/Wordpress is that the codebase becomes inflated and entangled when various third party apps are installed. It's quite normal for an eCommerce store to cycle through various apps during it's lifetime, but even when unused apps are deleted, some of the code remains strewn across the codebase.

All this means is that issues need to be solved by an adept coder and it's worth it to pay higher rates. I found a freelance coder, and with my experience as a front end web developer, I was able to manage the coder throughout the project. This included providing detailed diagnostics and determining which of their suggestions were worth implementing. This approach was effective, doubling the Google Lighthouse score from ~30 to ~60.

Ideally you want to be 90+, but unfortunately if you have an established website built with Shopify/Wordpress, the issues created by code from third party apps and the chosen theme can be too deeply entrenched to achieve that goal, and it wouldn't be worth it to rebuild the site entirely.