Wow, it's got some issues on Chrome (dropped frames on scroll) but Safari is another level. Selecting text takes time. Looking at Activity Monitor, I see "Safari Graphics and Media" using 200% of my CPU even at rest.

A quick profile on Safari shows some layout recalc happening regularly, but surely that shouldn't cause this bad of perf...

The last time I found something like this, it was because of 100's of box-shadows.

Edit: Sure enough, this cures Safari:

    *, *::before, *::after { box-shadow: none !important; background: none !important }

It's a combination of box-shadows and gradients.

Edit 2: Ah, they're using shadow DOM for the img reflection, so we can't affect it. Good gravy is the shadow DOM stuff overwrought, it's 87 elements all told, just for one img.