And I've been using iOS devices for just as long, and managed literally hundreds of units. Anytime I get a report of a 'slowdown' or 'poor performance' from one of my people, it has always turned out to be either user error or a hardware failure. On a properly operating iOS unit, I have never had a problem with the most current version available for that unit. None of my 3GS units ever had a problem with 6.1.6. None of my iPhone 4 units had problems with 7.1.2. And none of my iPhone 4S units (45 at this time, but previously over 200) ever had a problem running iOS 8, any iteration, unless there was a hardware problem.
Apple doesn't release an iOS for a platform that will not properly handle it. While an 'older' iOS may run faster, that's because the processor isn't being used to it's full potential. But an iPhone 4S still uses less than it's full potential (not my much, but close enough) when running iOS 8.1.2.