I have owned nearly every generation of Apple TV. Other than the original model, I wouldn't describe any of them as slow as molasses. On the contrary, they are consistently the fastest streaming devices I've owned, and I'm comparing to things like Roku (built-in to a TCL), FireTV, and Android TV across a number of devices (Shield Pro, Hisense U6G, Zidoo Z9X). (Technically the Zidoo uses Android, not Android TV.)

I currently own all three generations of the Apple TV 4K. All of them are snappy devices.

Now that said, I use them for streaming video only, both a variety of services as well as use Infuse for streaming from my NAS.

>they are consistently the fastest streaming devices I've owned,

As someone that works on set top box / streaming chips, I don’t think you realize how low that bar is :P

The prev gen ATV 4k is clearly superior, I’m not disputing that. But ‘snappy’ is a relative term. I expect a lot more out of a device sporting the chip it has. I regularly get input lag, stalled apps (that the system cannot recover from for some time), bursted input (like it doesn’t debounce queued remote inputs after stalling)

There are systems level issues that remain unaddressed, and I speculate they are due to hardware bottlenecks somewhere .

Yeah, I switched from a Shield to the prior gen of Apple TV 4K in part because the Shield was often laggy as hell (also the UI layout was a mess, also the damn ads, also it was significantly buggier) and AFAIK that's about the most powerful non-Apple box on the market. ATV's way better on that front. Frankly a low-end-but-not-quite-bottom-end Roku machine feels snappier and more-reliable than the Shield did (though not as good as the Apple TV)

Im a big shield fan, I don't feel like other boxes can touch its range of audio and video codecs. I did replace the default launcher with fLauncher so that I no longer had ads.

That sounds like you may have defective hardware or something else like an overheated environment forcing it to throttle. On a standard unit everything built-in is snappy.