Regardless of the speed of the machine, it will always be possible to create an image with enough heavy-duty filters to bog it down, even if Inkscape's algorithms were to be optimised to the max. I wasn't trying to suggest that a 500x500 pixel object will always slow a machine down, just pointing out that there's generally less work to do when it's zoomed out than when it's zoomed in. If an object has a filter that renders slowly - whether due to computational complexity, inefficiencies in Inkscape's code, or a slow machine - zooming out will likely cause it to render faster, and zooming in will likely cause it to render slower still.
Yes, a modern machine can fill the screen many, many times a second. But calculating what to fill it with can still be a multi-second operation, depending on the content.
Co-creator of The Greys
and Monsters, Inked