Delete comment from: Ken Shirriff's blog
Voltage sag on any charger is not really an issue as long as it doesn't sag below the maximum charge voltage of the battery it has been designed to charger - for the iPhone and iPad this is 4.2V fully charged. Which is below the sag voltage you measured before the current limit was hit.
Sep 12, 2013, 3:08:11 PM
Posted to A dozen USB chargers in the lab: Apple is very good, but not quite the best