From what I read, it does slow down your charging time. Not a lot but it does somewhat. Sometimes it depends on the quality of the extension.
I have a 6 foot extension added to my cell phone charging cord. Is my imagination or does this slow down the charging time?
I agree with Rooster, and also it seems to heat up my phone a lot, so I quit using them to extend my charger.
I usually end up on the floor with pillows under my head if I am trying to charge phone and talk on the phone at the same time. (Or at the same time as on here -I am on the floor right now charging phone answering questions)
Depends on the receptacle on the end. You SHOULD be purchasing one specific to your phone since it CAN regulate the current to your phone and battery. Using non certified plugs and cords could lead to early battery degeneration. Slow is better then fast when it comes to charging cords. It's not always convenient but it won't kill your battery as fast.
I just saw a story about prolonging your phone's battery life. There were several things I needed to adjust, like push notifications and screen brightness. Here's a link if you're interested.