99% of Shopify stores are loading slow. That's not because Shopify hosting is bad, they are loading slow because of the apps, code left from uninstalled apps or theme tweaks and most probably the themes are slow.
At https://shopyspeed.com we helped +30 stores to increase Google PageSpeed Score from 10/100 up to 90/100.
Liquid (the template language) is slow and extremely limited, and the apis are.. evolving. It's been hard at times to avoid nasty code that loops through everything to load a small chunk of data. At this stage we are starting to mirror Shopify content in addition to using it as a headless source.
Indeed, but you can still achieve good scores on Google Lighthouse. I've been trying Gridsome around 8 months ago with Shopify Storefront API and GraphQL hosted on Netlify.
To be honest, the page speed was the same as on Shopify hosting on the theme that I was building back then.
At https://shopyspeed.com we helped +30 stores to increase Google PageSpeed Score from 10/100 up to 90/100.