I recommend running your website through the Shopify Analyzer. It's a free tool we built for the Shopify community. If there are any major optimization opportunities, the Analyzer will let you know what they are and will give some actionable insights into how to resolve them.
Another optimization that Shopify stores often need is true lazy loading. Lazy loading waits for an image to come into a user's view before loading it. This way your website isn't trying to load all images at one time. We have a great guide on it: How to Lazy Load Images on Shopify to Make Your Site Faster.
Lastly, if you're referring to Google Page Speed Insights when you mention your low mobile speed score, I wouldn't worry too much about it. Most successful stores do not score well on their tool + there are a bunch of other metrics that go into how that score is calculated, not just page speed. You could actually have a very fast website and still have a low score. We have another article that outlines why we don't think Page Speed Insights is the best analysis tool for Shopify sites: The Truth About Google PageSpeed Insights for Shopify
I hope this info was helpful! If you need any optimization help, please feel free to reach out to our team at speedboostr.com/contact.