Compute once when the window is created or resized is what happened, yes, but that doesn’t work when using them for buttons (unlike MS Windows, where every control is a ‘Window’, on Mac OS controls inside a window didn’t have their own drawing context), or when using them for window content (e.g. in a drawing program)

And remember: the original Mac had about 28 kilobytes of RAM free for applications. The system unloaded icons, code and fonts that were available on disk all the time. Few objects were ‘tiny’ at the time.