Personally, this has been less of a problem as mobile device get more powerful. I don't worry as much about optimizing for phones. I worry about optimizing for search engines. That means I usually avoid large images.
So, that's two things right there…
ImageOptim for optimizing images… ImageOptim — better Save for Web …can make the file sizes of images smaller, either lossy or lossless.
PageSpeed Insights is another… https://pagespeed.web.dev …as it can point out lots of potential issues. Largest Contentful Paint (LCP) is an example, and it was previously discussed here… Largest Contentful Paint (LCP) / fetchpriority / SEO In Hype
Another thing is to simply use your developer tools. Even though you're using an iPhone, a Mac can be used to test things out.
Although, things like full screen animation, excessive animations, too many Physics objects, poorly written JavaScript, can slow things down. As an example, I've made games with Hype using SVG to keep the file size down. It works great when played at a small size. But when played in 4K, it starts putting pressure on the CPU. That's not ideal, especially with mobile devices, as that uses more power.
That's why testing with older devices is important too, as with Apple Silicon, my newer Macs are way faster. It's hard to notice performance issues when the computers are so strong.
Optimization is a moving target. Some problems go away, while new problems arise. I just realized that I have to look at my website as it's throwing an error. I didn't notice anything wrong, but I just ran a PageSpeed Insights test and it showed the error. 