slow download speed for facebook in some videos

[Wingzzzzz]

Checklist

• I've verified that I'm running youtube-dl version 2019.05.20
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.facebook.com/pomatohk/videos/2200508596651289/']
[debug] Encodings: locale cp950, fs mbcs, out cp950, pref cp950
[debug] youtube-dl version 2019.05.20
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.17134
[debug] exe versions: ffmpeg 3.4, ffprobe 3.4
[debug] Proxy map: {}
[facebook] 2200508596651289: Downloading webpage
WARNING: unable to extract thumbnail URL; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; type  youtube-dl -U  to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
[debug] Default format spec: bestvideo+bestaudio/best
[debug] Invoking downloader on 'https://video.fhkg10-1.fna.fbcdn.net/v/t42.9040-2/10000000_2380858885527557_3270492603765751808_n.mp4?_nc_cat=104&efg=eyJ2ZW5jb2RlX3RhZyI6ImRhc2hfdjRfaHE1X2ZyYWdfMl92aWRlbyJ9&_nc_ht=video.fhkg10-1.fna&oh=49ca83e81a6e5befa873dbbb21f311ca&oe=5CEA5446'
[download] Resuming download at byte 508863
[download] Destination: 【冰J大挑戰】  平時同朋友一齊唔知有咩好玩，學吓呀冰同呀J咁玩吓YouChallenge 嗰app嘅任務啦！  JefferyKwan - 阿J 阿冰 ...-2200508596651289.f2380858835527562v.mp4
[download]   0.1% of 742.08MiB at 47.82KiB/s ETA 04:24:35

Description

This is a case related to #21018
However, the case is locked and cannot leave on it.
Here, I would like to add some details on it.

1. Slow speed does not happen on all videos
   Slow:
   https://www.facebook.com/pomatohk/videos/2200508596651289/
   https://www.facebook.com/pomatohk/videos/300397234204420/
   https://www.facebook.com/ArmChannelTV/videos/2193152900997469/
   Fast:
   https://www.facebook.com/ArmChannelTV/videos/2225930714153863/
   https://www.facebook.com/ArmChannelTV/videos/2189253518052746/

For slow, it is about dozens of Kb(mostly 30-50),
while 30 Mb for fast.
(tests are done in the same period of time,
tried download slow videos first or slow later)

Cross checking slow videos,
the issue happens in cross channel.
By 3rd and 4th video,
there are slow and fast download speed in the same channel

2. Pattern of download speed
   Prior observation:
   Upload video = slow
   Live video = fast
   By 4th and 5th video,
   Downloading live video is fast for long or short length.

Key difference:
slow video with cdn:
https://video.fhkg10-1.fna.fbcdn.net/v/t42.9040-2/10000000_2380858885527557_3270492603765751808_n.mp4?_nc_cat=104&efg=eyJ2ZW5jb2RlX3RhZyI6ImRhc2hfdjRfaHE1X2ZyYWdfMl92aWRlbyJ9&_nc_ht=video.fhkg10-1.fna&oh=49ca83e81a6e5befa873dbbb21f311ca&oe=5CEA5446
fast:
https://scontent.fhkg10-1.fna.fbcdn.net/v/t42.1790-29/10000000_175061783441188_3251965317533690343_n.mp4?_nc_cat=110&efg=eyJ2ZW5jb2RlX3RhZyI6ImRhc2hfbGl2ZV9oZDFfZnJhZ18yX3ZpZGVvIn0=&_nc_ht=scontent.fhkg10-1.fna&oh=7182a387c065cadd823b18d2b1ecd2fe&oe=5CEA4D7A

While i can move along the time bar in the video in the web page to quickly watch the whole video,
i would say this is a extractor issue