2) the dev has 64gigs of ram and a newest CPU and doesn't care about performance issues for people on older computers... that's why you need gigs of ram just to read a weather report online.
People who don't like the developers work can always write and publish their own application, of course.
Honestly would not expect anything more from js devs who use macs/windows. I see this so often.
Is it worth it? Probably not, since this is a single-platform app to start with, but JS+HTML are easy to theme and customize, and Qt is... not quite as simple.
The documentation is the Objective-C docs, I use those all the time. You do need to understand the basics of how to translate from the Objective-C APIs to what JXA (or AppleScript) expects, but once it clicks you can do it for essentially anything with the same logic.