Slow performance from an old, weak battery.
Good. Very easy, very fast. Phone is super peppy again :)
Nowhere does anyone mention this but—you most likely need to recalibrate your battery after the replacement. If you don't, the battery percentage will most likely not display correctly. Thirty minutes unplugged might drop over 10%, even though it's probably really only 2%. To do so you have to drain the battery to below 10% (more accurate if you wait until it shuts off on its own) and then completely charge it uninterrupted to 100% (recommended to do this with the phone powered off) for at least 5 hours. Lastly, reset the phone by holding the home and sleep/wake buttons simultaneously, then you should be good to go. So you might not be able to use your phone for half a day or so, and if you charge it overnight while it is off, you can't rely on your phone to be your alarm clock like most people do nowadays. Just a heads up so you can plan ahead for this and so you don't think you got a dud battery.