Shortcoder is tested with 1000+ shortcodes and there is no issues/reports of slowness.
In your case 85 shortcodes is not a lot and the content size is small. I don’t find any reason for the page to be stuck for 8 seconds.
I created a shortcode for the JS you had shared and did not observe any slowness in my test site. The entire page loads instantly and the JS takes few seconds to load the “buy now” button. This is expected since thats how JS widgets asynchronously load content.
You can expermient with a normal HTML text with JS and see if there is any difference. Please let me know if you want any other findings or information related to this.