On what mirror are you redirected and where are you located?

Paste the output of "curl -sI http://http.kali.org/README" to help us.

I am from Bangladesh.

I am redirected to this mirror.

deb http://http.kali.org/kali kali-rolling main contrib non-free
deb-src http://http.kali.org/kali kali-rolling main contrib non-free

sorry I am redirected to this mirrors below

http://ftp.yzu.edu.tw/Linux/kali/README
http://kali.cs.nctu.edu.tw/kali/README

these are really slow.

@rhertzog I addressed this issue. I tested various mirrors by connecting with VPN servers of different countries. These are the mirrors that are offering very slow downloads.

http://ftp.yzu.edu.tw
http://kali.mirror.garr.it

The average speed i was getting from them was 15KBps. I'm sure there are more mirrors acting very slowly. Recently this mirror has also started offering slow download, average speed is 47.2KBps:

http://ftp.hands.com/kali

It used to be 300KBps and above.

Can you identify more mirrors which are offering slow downloads and refresh them?

yea why not I will post the slow mirrors list soon.

I'm sorry but the speed of mirrors depends largely on network connections between you and the mirror and it's impossible for us to measure speed as seen by users. We redirect to mirror close to you in the hope to give you the best result but when there are no close mirrors, the result might be disappointing. In that case, your best bet is to manually select a mirror that has a good connectivity with your and hardcode the mirror name in /etc/apt/sources.list instead of relying on http.kali.org.