iOS 9 comes with the capability to block ads when browsing using Safari. To start blocking any ads, you install one or more ad blocker apps from the App Store. However you need iPhone or iPad with 64 bit processor.
Apple A7 is the first 64-bit processor for iOS devices. At the minimum, you need iPhone 5s, iPad mini 2 or iPad Air in order to run ad blockers. On older devices, ad blockers are listed on the App Store but you’d get a “This app is not compatible with your device” error when you tried to download.
It is easy to see why Apple chooses to restrict ad blocker to the more powerful iOS devices. Ad blocker contains rules that instruct Safari on what to do while it loads a web page. One example of the rule could be to “remove ads when visiting http://www.spamsite.com”. The more ad blocker you installed (and depending on how many rules each one contains), Safari requires more CPU time to process the rules each and every time you view a web page.
Apple has designed an efficient ad blocking capability thus the performance penalty is negligible when running ad blockers.
It is early days as more ad blocker apps are expected to swamp into the App Store. If you have any recommendation, feel free to highlight your favourite ad blocker in the comments below.