Page Speed Insights show my site (on mobile) to be as low as 7, fluctuating up to 12.
1) Generally, how do I get myself out of the red, perhaps in two or three concrete things I can do? and,
2) Specifically: One of the biggest culprits on the Page Speed Insights "opportunities" suggestions shows to be "Defer offscreen images ---- 25.41 s". How do I do that or, is this something I would have to hire a developer for?
Also: there is a significant discrepancy between the speed results on Page Speed Insights versus the Speed Report (under Shopify --> Reports --> Behavior --> Speed). There I hit a 19...
Cheers and thanks again for any suggestions you may have!
PSI scores are generally low for ecommerce sites with apps or engaging features, that's just the nature of the tool (it's a general tool, not targeted for ecommerce or platform sites that use 3rd party apps).
For things you can do to optimize your site, check the Shopify Analyzer. That's a performance analysis tool my team and I built for the Shopify community, for free. It will tell you want can / should be optimized, based on our data from a couple thousand stores analyzed / optimized.
For your question #2, "defer offscreen images" is also known as "lazy loading". You may be able to implement yourself, I wrote a guide on how to do it: https://speedboostr.com/shopify-lazy-loading.
The Shopify speed score tool uses the same engine, but also factors in an average of templates, so your score can be different even though they use the same engine.
Each store facing app you use will affect load speed and scores in analysis tools (case study here), I recommend optimizing what you can, but don't remove apps just to get a higher score in any tool.