… Godox V860 …

Hopefully the II or the III version; the I version is more of a PITA.

I’m a little familiar with the concept of ambient light using a flash,

Just to refresh. Every flash photograph has two sources of illumination, and two different levels of exposure you combe in one shot.

The ambient (all the light that isn’t from the flash) exposure is controlled by iso, aperture, and shutter speed. This you should know. :)

The flash exposure is controlled by iso, aperture, power, and flash-to-subject distance.

Flash and ambient can combine where they overlap, so it’s not always as simple as flash = subject and ambient = background, but it primarily works out that way.

but here’s my question : If I don’t want to crank up my Iso for ambient light, how slow I can go with my shutter speed ?

It depends on how much motion blur you’re willing to have register in the ambient, either from subject motion or from camera shake blur. You can do a thing called “slow sync flash”: combine a very slow shutter speed with a flash burst to “burn-in” or freeze your subject.

The more you balance towards ambient, the harder it’s going to be to freeze motion. The more you balance towards flash, the more you can use the flash burst to freeze motion. The basic flash pulse duration for the V860 II is 1/300s (full power) to 1/20,000s (1/128 power).

The limit is more at the other end in how fast you can go. While a V860 can do HSS with the R6, it’s a game of diminishing returns. In general, you lose about 75% of your power (-2EV) if you go to high-speed sync (HSS).

… But with a flash, considering that it’s supposed to freeze the subject,

It can only fully freeze the subject if there’s no ambient light in the image (aka “killing the ambient”). That is, if your iso, aperture, and shutter speed are set so that when you take a picture without flash, you get a black frame.

In the ambient, you can still get motion blur from a slower shutter speed. And going over 1/250s (your sync speed), you’ll lose 3/4 of your flash power.