Most ecommerce sites score low with that tool, it's common if you're using 3rd party apps especially. Info about that here: https://speedboostr.com/google-psi-for-shopify.
Few tips to help guide your performance optimization quest:
1) Focus first on what you can reasonably optimize on a Shopify site. To see that, run your site through the Shopify Analyzer. That's a free tool my team built for the Shopify community - the scoring and recommendations are based on over 1,000 Shopify sites we've optimized.
You can use that in tandem with our free guide to optimize your site: https://speedboostr.com/shopify-optimization. If you have a developer share that with them. If you don't have a developer on the team, you should still be able to perform some of the optimizations yourself. We try to make it easy 🙂
2) After optimization, test your site on a slow connection. If you still feel it's too slow, you can look at either removing apps (not recommended if they're generating revenue) or invest in hardcoding store facing apps (integrate the feature directly into your theme code instead of relying on 3rd party resources).
3) "unused JavaScript and css" is good to take care of on your Shopify site if it's the result of wasted resources. For example, if you've installed a store facing app that inserted some code into your theme, and then deleted the app, unless you remove the code manually it will still be in your theme loading (because apps lose access when you delete them). This doesn't happen to all apps, but worth checking your theme.liquid file at least to see if you recognize any old app code. (we also have a thorough App Analysis service if you don't feel like digging in your code).
If you're talking about "unused javascript and css" inside your files, for example your theme.js file, that's going to be a major project to deconstruct your theme files and conditionally load only the code required for each separate template of your site. That will yield small gains but be an expensive project, probably not a positive ROI.
4) "Render Blocking Resources" can be misleading. It's good to defer resources that aren't critical to page load to the end of the page. But the challenge with a template site with 3rd party resources is that you can shoot yourself in the foot by deferring a resource you thought wasn't needed, but then actually is. You also can't control this if apps are loading the resources in the head (which often is by design).
For this one, I recommend loading any non-critical resources at the end of the page, but make sure you test thoroughly before publishing your theme.
You can implement techniques to load only the resources needed to render above the fold content, in the head... but again this is a time (or money) expensive process that also generates maintenance any time you change your site or sometimes install a new app. Usually not worth the ROI unless you're doing massive traffic volumes.
