How to watch video while still downloading including audio?

If I download a video using [youtube-downloader](https://github.com/yt-dlp/yt-dlp) I can watch the part file while downloading (in my case using [mpv](https://mpv.io/)) . Suppose I cannot or don't want to select a format containing both video and audio then the audio in the part file is missing because it is downloaded and merged after the video download is complete. Is there any fast way to get audio and video merged during the download, such that I can watch the part file including audio. I already asked a similar question on [github](https://github.com/yt-dlp/yt-dlp/issues/7767) and learned that I could use the --downloader ffmpeg option. This works, but is very slow, so I am looking for a faster way to do it. The problem occurs if I download very large video (for example 10 hours long) in a high quality. However downloading audio is much much faster. So suppose I have the audio file already and I am downloading the video file. Is there an indirect way (workaround) using for example ffmpeg to merge the audio continuously into the video while the file is downloading.