Is there a way to optimise the load speed through scriptTag api? or an alternative method to achieve the above?
Thanks in advance.
Don't make an external call simple. Upload your scripts to the Shop theme asset folder and include it in theme.liquid. Remember Shopify is server rendered HTML. So if you can get it in the templates there won't be much delay.
Do a PUT to the theme asset folder with your script. Add two scripts a small one that bootstraps the bigger one. Then update theme.liquid to include your init script. Finally in your init script append to the head the main script at the end of body load, something like that.
Init script allows the larger script to load after the HTML body has been parsed. Non-blocking, best practice. RequireJS with BackboneJS works the same way.