This is an example knowledgebase article to help troubleshoot why your iPhone is slow.
Your iPhone could be slow due to a number of reasons. Please follow the instructions here to determine the root cause in your case.
Step 1. If you have a new iPhone – such as 6S, 6, 5s, 5c, you are running an iOS that is iOS7 or later, please skip to Step 2.1.
Step 2. If you have an older iPhone – such as 5, 4s, 4 or 3GS & if you are running iOS4 or later, please follow the instructions below:
Step 2.1 Close unnecessary applications. To do this, Press the Home button two times quickly. You’ll see small previews of your recently used apps. Swipe left to find the app you want to close. Swipe up on the app’s preview to close it.
Now skip to Step 4.2
Step 3. If you have an even older iPhone – such as the original iPhone, 3G, or 3GS with iOS3, please skip to Step 4.2.
Step 4. If you have an iPhone 3G with iOS4 or a 3GS with iOS4, iOS5 or iOS6, iPhone 4 with iOS4, iOS5 or iOS6, iPhone4s with iOS5 or iOS6, or an iPhone5 with iOS6, please follow the instructions below:
Step 4.1 Close unnecessary applications. To do this, double tap the Home button to see all the background apps running on your iPhone. Now, press and hold down one of the app icons for two (2) seconds. The apps will start to wiggle and have a red minus sign on the left corner. Touch the minus sign on an app to close it.
Step 4.2 Clear Safari Cache. As you visit various sites using your iPhone, the Safari browser stores cookies and data (such as images from visited sites etc.) locally on your iPhone. iPhone checks against this data at the start of every website visit to serve web-pages quicker. However as this data grows with time & uses more resources, it results in a degradation in performance.
Step 4.3 To clear this data, locate the “Settings” app on your home screen & choose Safari. Now remove all history and website data. By clearing history, you will close all pages that you have open. Please save any unfinished work or bookmark pages that you don’t want to lose. Remember, one of the consequences of this action is that if there were any websites for which the login passwords were being remembered by your iPhone, they will now be have to be entered again! Confirm that you want to remove history and website data & proceed further.
Step 4.4 Perform a soft reset. Next, let us soft reset the iPhone. Hold the top lock button and home screen button until the phone turns off and the Apple logo appears on the screen.
Step 4.5 Mail, Contacts & Calendar Settings Check. Next, check & adjust the frequency with which iOS checks for mail. You can find this in Settings->Mail, Contacts, Calendars. Scroll down to find the “Fetch New Data” option. From here, you can adjust the frequency at which your iPhone checks for new mail. It also helps with the speed of battery drain.