download speed of url youtube-dl returns is very slow

[umlx5h]

• I've verified and I assure that I'm running youtube-dl 2018.03.14

• At least skimmed through the README, most notably the FAQ and BUGS sections

• Searched the bugtracker for similar issues including closed ones

• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'--extract-audio', u'-g', u'https://www.youtube.com/watch?v=dQw4w9WgXcQ']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.03.14
[debug] Python version 2.7.14 (CPython) - Darwin-17.4.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 3.4.2, ffprobe 3.4.2
[debug] Proxy map: {}
https://r6---sn-3pm7sn7d.googlevideo.com/videoplayback?expire=1521469765&gcr=jp&ei=5XSvWrLAKou3gQOh04qoAw&requiressl=yes&itag=251&keepalive=yes&initcwndbps=1402500&source=youtube&clen=3438483&dur=212.061&ipbits=0&sparams=clen%2Cdur%2Cei%2Cgcr%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&lmt=1518413814901302&mv=m&mt=1521448063&mm=31%2C26&fvip=4&ms=au%2Conr&pl=19&id=o-ANc6M-7es1U_ybk77INpHx-UBPu29oeLBUdfu508psBU&mime=audio%2Fwebm&gir=yes&c=WEB&mn=sn-3pm7sn7d%2Csn-oguelnle&ip=153.200.204.206&key=yt6&signature=3C110F464F336275AAB37309E69EB97DE5962E94.916C77A7815EE4CBF4AA43011AB023A58ADA7453&ratebypass=yes

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I want to create API server that responses youtube audio URL to let client download youtube audio file directly.
So I get youtube audio file URL using the following command.

youtube-dl --extract-audio -g "https://www.youtube.com/watch?v=dQw4w9WgXcQ"

And I got following audio url.

https://r6---sn-3pm7sn7d.googlevideo.com/videoplayback?gcr=jp&requiressl=yes&fvip=4&ei=smqvWqbHL8fkqQGgrKOQBg&lmt=1518413814901302&mime=audio%2Fwebm&mv=m&c=WEB&expire=1521467154&source=youtube&keepalive=yes&pl=19&itag=251&dur=212.061&initcwndbps=1396250&clen=3438483&ipbits=0&mn=sn-3pm7sn7d%2Csn-3pm76n76&mm=31%2C29&id=o-APcnop1e4uRZDEsrYtj_pmV3lsMsCTO4a0lmk4srZ4gm&gir=yes&mt=1521445365&ms=au%2Crdu&key=yt6&ip=153.200.204.206&sparams=clen%2Cdur%2Cei%2Cgcr%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&signature=E2078C6E348D3263DCDBF824190BC1BF2A938533.51541F2E85AFC8F3A6C0A142B37774DE16071692&ratebypass=yes 

And I opened this url in chrome and download it, but it takes about fifty seconds, about 60 kb/s speed.
Using wget command, results are same.

videoplayback?gcr=j 100%[===================>]   3.28M  64.7KB/s    in 52s

But for some reason, Without -g option, Download can be done immediately.
It takes about 1-2 seconds.

youtube-dl --extract-audio "https://www.youtube.com/watch?v=dQw4w9WgXcQ"

Why is there a difference in speed like this?
Both download url are same?

[Hrxn]

Expected.

youtube-dl uses downloading based on separate chunks to work around the rate limitation imposed by YouTube.