@2fd7551e not familiar with the technical browser implementation details, but if I understand correctly GPSI mentions "large DOM size" under "diagnostics" because CSS & JS performance is impacted by DOM size, regardless of elements being hidden? 

 @2fd7551e interesting! does HTML in a details tag not get automatically added to the DOM then? 

 @2fd7551e one trick could be to hide the HTML for the calendar  (in noscript tags) and only after a timeout (or visitor interaction such as scroll or mouse movement) unhide it with some JS, works like a charm :-)