This address previously carried a 2020 account of a high-speed-sync-versus-ND-filter demonstration by Gavin Hoey, written by John Aldred for DIY Photography. Photography Discover did not perform that test or take its example photographs. The original write-up and video are at DIY Photography. We have removed the reproduced account and its old retailer links.
What each method changes
High-speed sync (HSS) lets a compatible flash and camera expose a frame at shutter speeds faster than the camera’s normal flash-sync limit. The Godox AD400Pro manual notes that the flash’s effective range becomes shorter as shutter speed gets faster in HSS. Check that your exact camera, flash and trigger support the mode before planning around it.
A neutral-density (ND) filter reduces the light entering the lens. It can bring a bright ambient exposure down to a shutter speed at or below normal flash sync while keeping a wide aperture, but it also reduces the flash light reaching the sensor. Meter or inspect a test frame after adding the filter; the flash may need adjustment.
A simple exposure comparison
At the same aperture and ISO, changing from 1/250 second to 1/2000 second darkens ambient exposure by three stops. A three-stop ND filter at 1/250 second has the same mathematical effect on ambient light. It does not mean the flash will behave identically: the filter attenuates light through the lens, while HSS changes how the flash covers the moving shutter. Effective output depends on the equipment and setup. Our ND filter guide explains stop numbers and how to choose a filter for other uses.
Use the original demonstration for its particular results. This page gives the underlying choice and does not claim a new hands-on comparison or current product ranking.