I had a similar situation with that exact iPod and the original iPad mini. They just weren’t meant to handle iOS 9 and it was made significantly worse by aging batteries.

I’d say it’s definitely not as bad as it used to be, today’s devices are far more capable in terms of processing power. For the most part if a device can’t handle a new software feature they just don’t get it in the new update. I’m sure it’s not 100% perfect, and there’s bound to be isolated incidents, but I never had any issues with slow downs on my iPhone 6S or iPhone X (outside of battery problems, which were fully resolved once the batteries were replaced)

At this point the only reason I upgrade my phones is aging batteries and/or dwindling replacement battery support. That being said Apple makes official battery replacements a bit of a pain (which could be viewed as intentional to help encourage new iPhone sales).

The original reason I switched from android to iOS is because iPhone’s consistently work well and smoothly, all receive the same updates at the same time, and you’d get more updates out of them which helps them last longer. I just didn’t feel like dealing with the hassle of only getting 2 software updates on a major flagship (which was slowed by 6 months to a year by carriers having to apply their own patches) all for a phone that didn’t work too well to begin with.

Android has come a long way since then and I can pretty confidently say I’d be more than happy switching to a Pixel or Galaxy S phone these days. I’d even argue their phones are generally nicer in terms of design, and I love that they are more open for customization and other fun uses (ex. Game emulation, termux, mobox, etc).

The main thing stopping me is that Apple’s integration is just too convenient to beat. Everything syncs seamlessly between iPhone / iPad / Mac and it genuinely feels like they are extensions of each other rather than separate independent devices. Android just doesn’t offer enough for me to justify it over the Apple ecosystem.

That being said I do have an android phone I bought used on eBay for some of the fun stuff I mentioned above. I highly recommend it to any Apple users who don’t feel like fully switching to android