First, thanks for your kind words, and I've filed a bug report for your Preview bug.
Second, I can tell you that we've got both of your feature requests (replacing media without erasing from timeline, and keyframe options for speed control) high on our radar!
Third, to address your philosophical question: From a technical aspect, Apple has built many frameworks themselves that optimize the manipulation of Audio and Video - for this reason we are not currently working on an Android/Windows version of LumaFusion, as it would require us to build equivalent frameworks to use on those non-iOS platforms, which would take a very large team, and a lot of time. So these Apple Frameworks give us a lot of headway to work on further optimization of multiple instances of Audio and Video manipulation (which is how we accomplish 3 Audio and 4 Video tracks). Let this also shine a light on the equality between software quality and hardware specifications, while having quality hardware is necessary for smooth performance, it is not the end-all be-all for how smooth software operates (though we hope to see improvement on Apple's hardware to accelerate the performances gained from their frameworks). And maybe the most important bit is that we have a very experienced Head of Engineering who is able to push the Apple frameworks to their full potential, and then some.
Thanks for reaching out! We're really happy you enjoy the app
Keep us posted with any other bugs or features you'd like to see.