I have all sorts of chargers, Turtle, especially one with an extra-long cord (5 feet or so), because those ultra-short cords Apple includes in the packages with iOS devices are often inconvenient.
I have noticed no improvement in the speed of charging my iPad 1 or iPhone 4, no matter which charger I use. But perhaps I haven't noticed it because both iPad 1 and iPhone 4 charge pretty quickly for me, so I've never had to think about their charge-time a lot.
iPad 3 is a very different story. It takes a very
long time to fully recharge with the cord Apple included in the package (I'd say at least 5 or 6 hours from 2% to 100%), and none of my other adapters have managed to speed this up. It takes even longer
to charge if I attempt to charge iPad 3 with the iPhone charger.
Then it just crawls along, barely charging it at all. Whereas I've been able to charge the iPad 1 with my iPhone 4 charger without any issues.
However, if I recharge the iPad 1 or iPad 3 as soon as the battery drops to 59% or below (and I have the opportunity), then the difference isn't as dramatic, as when charging from all the way down. That's why I try avoiding battery levels such as 15% or 30% on my iPad 3, because the Retina-screen iPad just takes a lot of time to recharge fully.
I have no information about 2.1V chargers. If you can post a specific link, please do. Also, I live in Central Europe, and iOS chargers are likely different depending on which region of the world you live in. (When you visit Italy and Germany, two countries relatively close to each other, they both use different types of plugs. There are even more differences between continents.)