It doesn’t need to be realtime since you can pre generate an srt with time codes beforehand using something like bazarr. Whisper also runs faster than realtime in most model sizes, up to 32x realtime so it can really be worth it to add auto subtitles to media in your collection that’s missing subtitles as a one time job.
You can use bazarr to batch generate whisper subtitles for your Plex/jellyfin/kodi library: https://wiki.bazarr.media/Additional-Configuration/Whisper-Provider/