I'm Richard - CRO Expert at PageFly Landing Page Builder. Welcome to Shopify Community!
Regarding your question, you could give this app a try SEO Suite: Image Optimizer by AVADA Commerce and it is free of charge.
And as you already know, page speed plays a crucial role in conversion rates, therefore, at PageFly, when building your store using our app, we provide an option named LazyLoad in the Page Settings modal, which will help you to optimize the page loading speed and your pages will load faster. Moreover, there are some tips I wanna provide you with, and hopefully, they will help you speed up the loading time when needed.
Above is my recommendations, please hit LIKE if you find it helpful.
Best of luck!