Not the coolest answer but I feel like QT is gonna be hard to beat here. It has a ton of this functionality already, signals are not the enemy for something this complicated and time-driven, it runs on anything and performance is great, and its pretty powerful/flexible when you need to whip up custom graphics stuff. QML and JS are really nice to have. I wrote a modular synthesizer GUI with it ( https://github.com/ohm…
I'm not QT or any other gui toolkit is well suited for this. Every widget is custom and many require high framerate / low latency. It's probably best to treat it like a video game and just render everything every frame. Only use gui events to affect the underlying data model, don't trigger any gui updates or rendering from them. In this light you're looking for fast software or OpenGL rendering as the key feature in…
But that's the thing, unless you are _recording_ audio, you can cache and pre-generate the visualisation before hand. FFTs even on 24 192khz 24bit tracks isn't that taxing.
Even visualisations don't have to be that low latency. So long as the audio tracks are in sync you'll mostly be fine.
For waveforms you can literally side scroll the visualisation. You're not going to be pushing more than 60-75hz which gives you 13ms per frame at worst.