Huh. Opening this webpage on Firefox floored my laptop (8 core 16GB). The lag was several seconds, including for clicking "back" or opening a new tab.
Might there be a problem with your device?
It's light grey (#666b67) on dark grey (#222623), not much contrast on desktop. Mobile uses other colours, the same background (#222623) but a lighter font color (#B2B5B3), which is _way_ better.
Why not use the same foreground color on desktop?
Sadly the font colour on non-mobile devices is way too dark, the whole site is way too low contrast: #666b67 (desktop) vs #B2B5B3 (mobile) on #222623.
Desktop colours: https://webaim.org/resources/contrastchecker/?fcolor=666B67&...
Mobile colours: https://webaim.org/resources/contrastchecker/?fcolor=B2B5B3&...