If I had to guess, it's that Apple refuses to implement very CPU-heavy JS features they don't think add much value in Safari.
I recall some feature I was using that ran in an inferior way to save power on Safari, so much so that I had to kill that feature and use a different API
I think it's more that Apple implements power saving behavior into the JS engine on desktop much like they do on mobile. For example, pausing background scripts on inactive tabs and disallowing autoplay on videos even on desktop.
I recall some feature I was using that ran in an inferior way to save power on Safari, so much so that I had to kill that feature and use a different API