Scrolling Block running slow on mobile

Hi all . Ive added scrolling blocks on the trial site I'm doing for a family member and they work very well on desktop and mobile on the Sqspace site. However,  when I share a preview of the site to those interested the scrolling blocks viewed on the mobile (iphones 10 upwards)  show the blocks running at crawling , jittery speed, whilst everything else runs well including video content. 

Any advice very gratefully received