Downloading speed is way too slow like 20mbps

It may be a busy download server or busy nodes anywhere between you and the download server.

There is always a concern that the speed limitation may be caused by your local Wi-Fi connection being awful. So before you go much further, see what your speed is when you run a download speed test. Just enter "speed test" in to any Browser search window and pick one, and run it to see a typical best-case speed under current conditions.

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When the Apple downloader starts up, it looks for two things: An estimate of how fast your connection might be, and whether it can combine your download with someone else's download to save overhead at its end.

If you don't like the speed at which it will be downloaded to you (as indicated by time in days) cancel it, and ask again in a few minutes. In many cases, the answer will be different.

Here, enter this Open DNS servers into your network setting.

208.67.222.222

208.67.220.220

Go in system preferences < Network < Advanced < DNS tab and Enter these servers in the DNS servers list.

Quite Safari, restart your macbook and try downloading OS X El Capitan again.

Hope this helps.

A different DNS server may get you connected to a web site faster the FIRST time, but for downloads, the first packet time is not the limiting factor. For something the size of the El Capitan Download, 'which DNS Server you are using' will not make a measurable difference.