What are you exactly meaning by sag? Voltage drop?
Its when you fire it and your battery meter on the screen drops drastically. It happens especially when
batteries get below full charge, and some
batteries will do it more than others. Some will sag all the way to the bottom when they are half charged, and when it sags that badly it effects performance. If I remember right, voltage drop is from the internal resistance of the
mod or low conductivity in connections, causing a loss in voltage, that applies more to mechs than to regulated mods I think.
Battery sag has to do with the battery and amps being drawn, I believe. It might result in voltage drop as well, but that shouldnt affect a regulated, so im not sure how it hurts performance,. I'll have to research it some more.
If anyone knows more about this than me, feel free to take over for me or correct anything I may have gotten wrong, I'm certainly not an expert.