To be honest, I might be mistaken, but a whole lot of the Jupiter theme css and js files are “designed well enough” for the objective you tend to achieve, being the absence of “unnecessary site files”.
Sure, there might be some improvements that can be realised…
… but that is not the issue at hand nor the solution for the problem at hand.
The issue at hand is: WordPress…with a huge number of “fat” plugins that are not coded properly.
Consider WooCommerce as an example: all relevant site files (js/css/others) are loaded, from the main page already, irregardless of the question whether one uses Woo elements on the main page.
Is this bad? Yes and no.
Yes, it is bad… if and only if one does not use Woo elements on pages on which the concerning site files (js/css/others) are loaded.
No, it is not bad … even if one does not use Woo elements on the pages that load specific site files.
One important solution in the case of performance considerations is browser caching.
And from that perspective, it is highly recommended to (lazy) load scripts, css and others as soon as possible, which would be “as soon as the site is loaded”.
Simply because of the fact that browser caching makes any reloading of site files unnecessary.
Nevertheless, one sometimes wants to reduce the number of files loaded on a specific page.
And that can be easily done by tweaking the functions.php file (or, even better, a custom functions.php file, which is highly recommended).
One of the problems at hand can be hence be solved by browser caching.
However, that is not the best solution, any proper solution is a myriad of measures that contribute in a specific way to boost the performance of the site.
For instance, simply using Apache is a problem: one really wants a proxy like Nginx to serve static content directly and/or to cache specific dynamic data, rendered by php scripts.
In short, consider to use a proxy like Nginx.
In addition, consider to use a CDN whenever possible.
Cloudflare is a good solution, but Verizon or Akamai CDNs are to be preferred.
Finally, never forget that we are using Visual Composer, which brings a performance penalty.
In conclusion, there are many ways to solve a problem, but first pick the right problem!!!
The problem is not Jupiter, it is WordPress, it’s plugins and the design (i.e. coding) thereof.
Sure, Jupiter is not very lean, but it is not supposed to: if it would be intended to be lean, mean and very efficient, one would not even have all the conveniences of this excellent theme.
In essence, if one really wants to boost performance: stay far away from php, only use plain-old html (static) and never use Apache (but use Nginx as a light-weight web server).
But we cannot do so, meaning that we can only lighten the pain by aiming for caching and disabling some site files (js/css/others) every now and then on specific pages.