I think HTML, CSS, and JS could be the answer here.
However it relies on two important things:
1. Using something which uses the OS's built in browser (rather than something like Electron).
2. Keeping your JS very lightweight.
This was shared a few days ago, combined with py2app could be a winner:
>Something that when the rest of someone's computer is slowing down, this can stay responsive.
Yeah so without having to overwrite some very deep OS settings, which will likely raise some red flags for your app requiring administrator privilege to run, you aren't going to get this. If someone has a browser thats taking up most resources, its gonna throttle everything.
There is generally very little reason to write anything but web apps these days for things that don't need to touch specific hardware. If you stick to as much native js/css/html as possible (i.e avoid large libraries), it will load blazingly fast and use way less resources then web apps that are considered fast.
Give https://sciter.com/ a try. It's fast and small.
If you want the fastest cross-platform native app. Build a native app for each platform and have the design teams review each other's work so everyone stays roughly on the same page.
You're not going to hit your responsiveness goals otherwise. And it's not going to feel native otherwise (if anyone actually cares about that anymore, I dunno)
Linux/Windows/MacOS: https://github.com/libui-ng/libui-ng
Lazarus/Free Pascal, I've run the IDE on a Raspberry Pi Zero W, it wasn't anywhere near as fast as I'm used to on a PC, but it got the job done. It makes small compact efficient GUI and command line applications.