Performance: changing a setting that affects text is slow

Changing any of the settings that affects text rendering (font size etc.) is slow. After investigation we know that this is due to theme updates and updating the UI layout in the application. The profiler does not give information about what is happening, so what can be done still needs to be investigated.

It's already been mitigated in part by removing many references to the theme resource, which caused excessive re-layout operations, but it still takes upward of 500ms which is way too much.

- [ ] Investigate the root cause and where the time is going
- [ ] Plan changes to avoid this freeze