It's a fact - older iPhones are hardwired to slow down after a while. 

The official statement reads: 

Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices.

Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components.
We knew that over time they got slower. 

But now it has emerged that Apple have been deliberately slowing them down during updates. 

Not cool. 

What can you do? 
Well, there are a few ways you can extend the life of your iPhone, short of getting a new one.

1. Stop charging your phone overnight
As indy100 previously reported, your phone battery depreciates from the first charge. One way to prevent it from atrophying is limiting the amount you charge it. 

2. Replace the battery in your phone
This could void your warranty, but would only cost you around £60, which is much cheaper than buying a new phone.

3. Buy a cheaper phone. 
You could forgot getting an iPhone altogether. Because having a device that is made to deliberately decline in its performance is not something you want, anyway.