This video was the 1st time I've heard that there's an initial cost to charging railguns.
I don't see anything that indicates that this is the case in the GSF interface in game, Dulfy's calculator, patch notes, or the Stasiepedia.
I tried to look for such a drain on weapon power in the video, but I'm not seeing it.
It's possible that it's small enough, something like 5-10 power, that it's not something that can be easily noticed just watching the indicator, but large enough to be noticeable after a few shots.
2 cases where it's clear that a snap shot is superior to a full charge are to avoid overkill and on a target that's about to LOS or move out of range.
The former, overkill, is just wasted time and energy, as Lompi said.
In the latter, the alternative is doing no damage to the target, because the target can no longer be hit.
The use for gunship duels is something I hadn't really considered before.
So far my strategy for that has been full charge ion, then slug, or 2 full charge slugs, but it sounds like getting in a partial charge ion earlier for the regen debuff may be better for cases where both gunships are already aware of and aiming at each other.
If you're trying to fit partial charge shots of the other railgun in between shots, the extra damage that can be done is 0.9/2.7=1/3 of a full charge.
1600*0.9/2.7 = 533.3333 slug
1760*0.9/2.7 = 586.6667 slug w/ 10% damage
1850*0.9/2.7 = 616.6667 ion vs. shield
463*0.9/2.7 = 154.3333 ion vs. hull
I'm assuming 0.9s for both reload time and time between switching railguns, with Rapid Reload.
There are definitely cases where this could be useful, since otherwise, you're just waiting for 0.9s for the reload time anyway, so if you have the energy, you might as well fire an extra railgun.
There is one case I would definitely not do this: when I have DO, I'd use slug only, so ion AoE does not warn other enemies there that there is a threat.