Why is apt-get downloads so slow

It seems to be a problem with the sources. Go to

System Settings -> Software & Updates -> Download from -> Select best server

For more additional details, refer this thread.

The apt-fast package can be installed in all currently supported versions of Ubuntu by adding the apt-fast/stable PPA to your software sources and installing it using these commands.

sudo add-apt-repository ppa:apt-fast/stable 
sudo apt update
sudo apt -y install apt-fast 

During the apt-fast installation process, you will be prompted to perform some package configuration. After successfully installing apt-fast, simply use it the same way you run apt or apt-get.

In my case, I had set a bandwidth limit in /etc/apt/apt.conf.d/50unattended-upgrades:

// Use apt bandwidth limit feature, this example limits the download
// speed to 70kb/sec
Acquire::http::Dl-Limit "70";

(I wasn't expecting this to affect my "attended upgrades" also..)

As soon as I commented out this line, apt install went much faster.