Hi Guy,
To Measure the enhance of the performance I would do the following.
1- Select a page where you have your videos and run tests o GT Metrix and Pingdom or whatever tool you choose.
2- Save those pages as Cornerstone templates - so you have them saved and avoid re-working if you decide to keep the current design.
3- Change your embed videos for images which will launch the lightbox.
To do that, add an image, link it to the video.
Add a class to the image lightbox-video-1
Add a raw code element and add : [lightbox selector=".lightbox-video-1"]
Save it, check if the lightboxes are working, if you have more then 1 lightbox use multiple lightboxes selectors on the same raw code element with different launcher classes.
Like that you will see what the performance enhancement you will get out of it.
Hope it helps