Hi,
I don't see why it would help in anything regarding SEO or speed.
Joomla only loads what's need on a page.
So if you have a forum extension on your website, it won't (it should not) affect the pages of HikaShop, and vice-versa. And thus having many extensions on a Joomla website should be possible without affecting speed that much.
I suppose this could be a wiser recommendation for WordPress websites, where all the plugins are loaded on every page.
Now, I'm not an expert regarding SEO so maybe I'm missing something ?