Why is vertical scrolling (seemingly) faster than horizontal scrolling? [Details inside].

It has to do with :
– the the kind of graphics subsystem is on the machine,
– combined with how well the operating system takes advantage of the capabilities of that graphics subsystem,
– combined with how well your browser takes advantage of what your operating system has available.

Some systems will have hardware acceleration for almost anything you would do with an image, and very good drivers for the operating system to hook up with, and a decent browser will make the right call to use all those.

Other systems might have mediocre hardware support, with mediocre drivers, and maybe the browser you’re does not take advantage of whatever better way to move horizontally there may be.

After some (very brief) experimenting… the speed of the scrolling seems to depend on the ratio of the length of the scroll bar compared to the total length of the window. If the scroll bar is only just smaller than the window you get slow scrolling, if the scroll bar is only 10% of the window then you get faster scrolling. Due to the widescreen ratio of most modern monitors they are much longer than they are tall so the scroll bar on the horizontal is always a much smaller percentage of the total image than the verical scroll bar. Adjusting the window so both scroll bars are equal (ie not using the whole monitor) the scrolling should seem more equal. But then you lose a lot of monitor real estate.

Also if it’s a really big image file, the speed in any direction could also have to do with the way the browser caches the part of the picture it is Not showing. That could be optimized for vertical scrolling on your rig.