I don’t believe the A8 was massively under powered, I think the struggle was because of one GB of RAM. The same A8 chip that’s used in the iPhone 6 is also used in the HomePod. Especially the 6 Plus was the worst experience with only Having 1 GB.
It wasn't massively underpowered, i agree, however It was just not a big upgrade from the A7 or the iPhone 5s internally. The A8 is really a smaller A7, most of the performance updates were in the GPU. The CPU in the A8 is more or less identical to the one in the A7.
At its time it was still the most powerful device around
1GB of Ram didn't help but that same argument could also apply to the 5s. Even in 2013 1gb of Ram was tight for the 5s, let alone the 6 the following year.
The 5s and 6 used the exact same type of Ram as well, not just the same amount. The front cameras identical, the rear cameras very similar. They really were very similar devices and there's nothing wrong with that. It just goes to show how advanced the A7 was in its day. I do agree the A8 gets unfairly viewed because of that, however the A8 was also a "perfect" version of the A7. The iPhone 6 did have some other features, design aside, that are modern like the NFC and wifi ac support
The A8 is way more power than one needs for the functions a homepod performs. I believe the A8 supports "hey Siri" when plugged into an outlet so that works well from a cost perspective I suppose.
Even today the iPhone 5s on iOS 11.4 runs pretty well, and works perfectly fine as a smartphone (I'd say the same about the 6), it's just people have become accustomed to newer devices and sometimes view these older ones unfairly. That, and iOS 11 was bugriddled at release - Apple has really improved performance on the last few versions for older devices. My 6 runs great now