Apple has just started pushing out the update to iOS 8.1.1 to all supported devices. Every Apple device that supports iOS 8 and iOS 8.1 will get the iOS 8.1.1 update. Depending on the model of the product you're applying it to, the size will vary - but it will be around 64MB. According to Apple, the update "includes bug fixes, increased stability and performance improvements for iPad 2 and iPhone 4s". Owners of older Apple devices sometimes complain that after upgrading to new iOS versions the performance of their gadgets heavily decreases. In the case of iOS 8 (or at least iOS 8.1), this has now pretty much been confirmed by Apple itself. So if you own an iPhone 4s or iPad 2 running iOS 8.1, you should probably apply the update to iOS 8.1.1 as soon as possible. Hopefully it will restore some lost performance on those two...
Source: http://www.gsmarena.com/ios_811_now_rolling_out_has_fixes_for_iphone_4s_and_ipad_2-news-10233.php
