Set temporary download directory prior to merging

[kik4444]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.07.28
• I've searched the bugtracker for similar feature requests including closed ones

Description

When downloading most videos, youtube-dl downloads the video and audio stream as separate files and uses ffmpeg to merge them, correct? This might take quite a long time on larger videos and quite disk intensive since the disk is both reading from and writing to itself.

May I request a feature to set a temporary download directory for the separate video/audio streams before they are merged into the directory specified by the --output option.

My primary storage location is an HDD and I believe there would be a noticeable performance increase if a video's temporary files are downloaded to another disk, such an SSD, and then moved to the HDD during ffmpeg's merge process. This way the same disk doesn't have to both read and write to itself.

For example, youtube-dl --tempdir ~/.cache --output /mnt/Movies/%(title)s.%(ext)s could download the video and audio streams in ~/.cache, which is an SSD, and then merge the final video in /mnt/Movies, an HDD.

I checked through the readme and bugtracker, and I did not see such an option. Apologies in advance if I missed it.