When I am using youtube-dl recently, I found the speed drops a lot. It was 5-6Mb normally but recently it’s just half megabytes. The solution is that use aria to split the download thread, which will call aria2c. It needs to install aria first, then add the following code to the original youtube-dl command:
--external-downloader aria2c --external-downloader-args "-x 16 -k 1M"
In which after -x is the splits number and -k obviously the block size.