I'm pretty sure the new iPod touch would still feel significantly faster despite being on iOS 8.4.
Significantly faster?
I use my iPod for mostly music and light media (I don't really use iOS games on my iPod because the screen is too cramped for that now, I use the iPad for that)
My iPod Touch 5th Gen on iOS 6 loads most of the stock apps around second or less.
Specific apps I use on a daily basis...
The Camera comes up in 1.6 seconds.
Music comes up in 0.8 seconds.
Settings comes up in 0.6 seconds.
Facebook comes up in 4 seconds.
Instagram comes up in 1.8 seconds.
Twitter comes up in 6.7 seconds.
Bootup time is 27 seconds.
I mean how much faster can it actually get at this point? Bootup time and initial Twitter launch will probably see the most improvement, but if it can't cut all these times by half or more, I wouldn't call that a significant increase.
I'm comparing the iPods on the OS'es they were
designed for...
If it was iPod Touch 5th gen on 8.4 vs 6th gen on 8.4, then yes, it would DEFINITELY be significantly faster.
I don't think that's the case in my comparison. That's the whole point of me not upgrading to iOS 8 in the first place. What's the point? All of the apps I use are still supported under 6. And like I said initially, it'd be "as fast" as a 6th Gen under similar conditions, since it's not running an OS that unnecessarily make the device crawls to its knees (which won't be the case for the 6th gen).