I recently took to rewriting what should be a very simple app from Obj-C to Swift with SwiftUI - because it's the future. The CPU usage was at 5% while idle, just for having a simple tiny pie chart that updates. Not to mention that for some seemingly basic things I still had to use AppKit anyway.
Wrote basically the exact same thing 1 day later in Swift with AppKit and NO SwiftUI and it sits at 0% CPU usage with less code complexity. Maybe in a few years I will give SwiftUI another try.
Did you find out what was causing it? I have developed 'heavier' (aka 50k lines of code) applications in SwiftUI on mac and it mostly sits idle (0-1% cpu based if it is doing some regular background stuff). Heck, I just created a quick Charts based app from an online example on Mac and stays at 0,0%.
Is it possible it is re-rendering the view hierarchy due to some data invalidation you haven't noticed?
Was this with the debugger attached? Did you do any further profiling in instruments on a release scheme to determine the cause of the cpu usage? You should be able to narrow it down to the specific sys calls.
Wrote basically the exact same thing 1 day later in Swift with AppKit and NO SwiftUI and it sits at 0% CPU usage with less code complexity. Maybe in a few years I will give SwiftUI another try.