My input, please entertain with a grain of salt.
A (proper) flash has varied output of "power". You adjust the power and it fires once at your camer's sync speed (or below).
For HSS, the flash does not fire once, it fires in a burst. Even an expensive studio strobe that does HSS will fire in burst.
What happens is the "power" of these sequence of flashes are reduced in order to fire in a strobe and cover the full
sensor with even light.
It is asking a lot of strobe tubes and supporting circuitry to fire FULL POWER in a rapid secession of flashes.
For those that are unknowing, the burst cannot be seen by the naked eye, so it "looks" like one flash...
except to the camera's sensor.
Therefore, given the nature of HSS, you cannot achieve max power output.
Using HSS, you adjust your shutter speed limit the amount of ambient light. This allows you to darken the exposure
of the subject and the background, then using flash you highlight the subject.
As we know from Photography 101, when shooting flash, either by x-sync or HSS, ambient light is controlled
by shutter speed and flash intensity is controlled by aperture.
For someone (like I was a few months ago!) that does not have HSS capabilities (both a camera that supports
it, and a flash that will go into HSS mode), similar effect can be achieved with an ND filter.
Using an ND filter, set the camera's shutter speed to x-sync and adjust the aperture to the desired F number
for bokah, etc. Select the ND or adjust the VND (my pet) to the desired overall exposure that you are going
for, typically a darker background with a flash illuminated subject.
The final step is to adjust flash "power" until the subject is illuminated with the desired effect.
Below, please see three images that I shot prior to having HSS, utilizing X-Sync and a VND.
As you can see, the results are very pleasing (IMHO).
However, when you try to use the same system indoors, limited ambient light creates a problem
with you AF system, as I quickly found out, with much frustration. I finally got some of the shots
I wanted, but the process was not joyful.
In this, and other similar shots, AF had to struggle though the VERY dark VND.
When using an ND filter, it is noteworthy that you are killing both ambient and flash illumination!
There fore, you must increase flash "power" to compensate.
In the end, regardless of the method you use.. you will never recognize full flash power!
Predictable with practice
Limited flash power
AF struggles with indoor light
"Easier" to achieve desired effect
"Faster" to get the exposure you want
IMHO easier to predict exposure settings
AF is not inhibited by reduced light though the lens
Camera must support it
Flash must support it
In the end, experiment with both.. But from my experience, I am so glad Ive upgraded to HSS!
This may be very elementary for most here, and it may be riddled with technical errors, however
I am confident the spirit of the message is concrete. I encourage corrections where applicable.
If I only helped 1 person with this post, my duty has been done.