Don't forget that his perception of iOS 9 could be different to yours. He might be the kind of person that couldn't care less about waiting 10 seconds for an App to open and respond. He probably taps on an app and goes for a walk and comes back later.
Also, he turns off all animations, so his perception of animation lags could be no existent.
Also, he could just mean "It's good enough", not meaning that it's a great OS, but rather it "sort of works" and it doesn't matter much to him if the entire animation is missing or that it takes 3 or 4 taps to close something because it's lagging so hard.
My wife is like this as well. I'll go on and on about how laggy and glitchy iOS 9 is and she has no idea what I'm talking about. She's content with it and just doesn't care enough about electronics to even notice stuff like that. She also doesn't notice screen tearing in video games (GASP!). I'm like "Look, you don't see those giant lines going through the screen 80% of the time?!?!" I wish I was like her, but my brain must be a ninja or something... I dunno.
8.3 was better than 8.4.1. The new music App in 8.4 SUCKED and it even lagged the phone (a preview of what was to come in iOS 9??)
Also, you've probably become so accustom to iOS 9 that you've forgotten what 8.3 was like. I know I have. It's hard to remember how smooth iOS 8 was when you no longer have iOS 8 to compare iOS 9 to.