People are making it seem like an iphone that is a few years old is so slow that its unusable. Reality is that this decision really extends the life of the phone for most. A phone is SIGNIFICANTLY less useful if you do not have the battery to keep it on all the time. A slow phone may be annoying at times, but its not like its functionality goes out of the window.
Imagine if they did nothing to extend battery life, people would be complaining about manufacturers putting crap batteries in, forcing people to buy new phones.