- Cheap range/target ammo is much "flashier" than premium self-defense ammo, as the cheap stuff has no flash retardant added to the gunpowder.
- Flash is usually only visible under low-light conditions, such as indoors or at night.
- Ported/compensated autoloading handguns are less "flashy" than magnum revolvers with full-power ammo, or even some non-magnum revolvers with less expensive range/target ammo.
- The flash is very brief and dispersed; it's not like someone shining a bright light in your eyes, or even as bad as a flash attachment used on a camera for indoor photos.
So, when shooting outdoors in daylight, no problems at all.
Shooting indoors or outdoors at night with premium ammo, again, no problem.
Shooting indoors with cheap ammo, flash will be noticeable but manageable. Honestly, I sometimes notice it in my peripheral vision, but I do not lose sight of the target for any longer than it normally takes for the weapon to cycle and get back on target. Single shots or the whole magazine, it doesn't matter. If you study the G19C photo, below, you'll see that the flash doesn't "bloom" until it's well above the weapon; in the line of the sights, there is very little to see under any conditions.
Glocks have had ported options ever since their first generation of handguns. I have a ported 1st Gen G17L, I have seen several 2nd Gen ported .40s, and I own a 3rd Gen G17C and G19C. I used to own a .357 G32C, got rid of it due to ammo costs killing my budget. Here is a photo of the 9mm G19C firing, taken on an indoor range, with the flash backed by the black backstop for visibility, using cheap ammo:
Contrast that flash, with this .357 shooting full magnums OUTDOORS during a pistol match:
And then there's my .44 Mag Redhawk with very flashy handloads (again, outdoors in full daylight):