According to this article by Apple Insider, iOS 9.2.1 brought speed enhancements to older devices that rivals iOS 8.4.1. I know this forum has been very critical of iOS 9's performance, so I'm curious what people think about this.
Perhaps Apple is addressing this issue.
Although installing 9.2.1 still causes an iPhone 4S or 5 to boot slower, once in action the software can load apps as fast or faster than 8.4.1 in many circumstances, according to iAppleBytes. Boot times are said to be speedier than 8.4.1 when running the software on an iPhone 5s, which was also the first iPhone with a 64-bit processor.
Perhaps Apple is addressing this issue.