Hi Coen,
SEO: About the only difference is Elemntors lack of Schema markup. Something which they may be addressing (community requests have been ongoing). Most noticeable is its absence in their nav widget.
Speed: Negligible if you’re using Elementor on the page content as well, the additional overhead will be present anyway. Most of which can be reduced with optimization.
Glad to here you’re enjoying GP and WP Show Posts is awesome, combine it with a little bit of HTML / CSS or a grid plugin and i sometimes question the need for a page builder ๐