There's a feature with flashes that if you're in the know, is a lifesaver. High speed sync is something that allows you to shoot beyond what's considered the maximum sync speed of your camera, and to shoot wide open with flash in bright light, it's often a necessity to balance the exposure.
Most cameras allow you to sync a flash at a somewhat slow shutter speed. For most cameras, that's around 1/200 or 1/250 of a second. This means that you can fire the flash with the maximum amount of power up to that shutter speed. But depending on the speedlight or flash unit you are using, it's sometimes possible to push the light beyond that shutter speed by "pulsing" the flash to work with the shutter curtain. However, this results in a loss of power.
For Francisco Joel Hernandez, of FJH Photography on YouTube, that's a price that's too high to pay. In examples shown in the video above, he argues that the power loss with flash is huge and that you often end up carrying around huge flashes and using them at much less power than they are rated for. As a better solution, he offers up ND filters as an option. That means by lowering the exposure at the lens instead of using shutter speed, you can keep that wide open aperture and also keep the shutter speed within the sync range as well. Win-win, right?
In my own experience, I'd say that it's a definite "sometimes." I've gone back and forth over the years between using ND filters and high speed sync on my flashes, and I can honestly say there's no right answer. Often, a complicating factor is the system I'm using. For instance, I would often use Cactus flash triggers to get cross-brand wireless flash compatibility between speedlights and camera bodies of different systems, such as Canon to Nikon. However, this often meant that high speed sync was off the table, and so, in these cases, an ND filter was really my only option.
Other, I'd find my camera's autofocus system to be a limiting factor. Hernandez, for example, lists a 6-stop ND filter for this wonderful photo on his Instagram page. I've tried to shoot moving kids with a Canon Speedlite 600EX-RT and such a filter, and even a decent autofocus system like the one on my Canon 6D would just give up in a lot of conditions with that kind of loss of light. That said, it looks like Hernandez's models moved a bit slower than squirrelly children, and his Sony Alpha a7R III's autofocus system hit the mark.
What are your thoughts on high speed sync versus ND filters? Is one better than the other? Leave your thoughts in the comments below.