Lightbox opens/Vimeo stutters on launch...

Site URL: https://bell-parrot-x7ly.squarespace.com/

Hi there, 
Having an issue on a site i'm struggling to resolve, & worse explain to a client. 
When you launch a video it 1st lightboxes the thumbnail, that fades down, then jumps to the Vimeo & occasionally there's a flicker or stutter on the Vimeo video too. 
It's a fairly jarring way to launch a video & struggling to find a solution. 

I've deleted all CSS to check but made no difference. Any advice would be really useful. 

https://bell-parrot-x7ly.squarespace.com/
Password: boomtown

Also included a video of the issue 

Thanks