Welcome to Shopify Community.
Hope this message finds you well.
Regarding your question, perhaps it is the product page that is slow.
After all, Instagram immediately opens the product page in its built-in browser. And we don't think that's the problem.
It's best to check the product page. You can share with us the URL of one of your products and we will do an analysis for you.
There is a chance that your store is loading slowly on mobile devices.