Hi @vojtechzikmund,
The theme’s font is not directly integrated to lightbox font, it has a separate implementation. Plus, it’s connected to post content through Ajax, the reason it’s slow and the other parts are stripped away (like header and footer where fonts are declared).
And with ajax content, the lightbox downloads it so it will appear slower compared to contents already available upon page loads (like images). If you wish, you can also apply the font through CSS,
Example,
.eg-lightbox-post-content {
font-family: Arial;
}
As for slow loading, there are many contributing factors like the internet, content size, optimization, and so on. There is no workaround for that setup as of now.
Thanks!