Hi there,
I just took a look at the script we use for smooth scroll and I’m not sure what could be causing that. It could come down to how the browser calculates the time you’ve set – I couldn’t see a solution, unfortunately. Does the same thing happen on a blank page with nothing but text?
It also looks like the script is no longer being maintained as there’s a CSS-only method that’s widely supported now (scroll-behavior: smooth). We may opt to go the CSS route in the future instead of JS, which means things like the scroll duration would likely go away.