I was trying to install some applications when I saw extremely low speed of downloading. I had a fast connection and browsing was fast. Therefore, there was a problem with the synaptic.
To solve this:
Go to System -> Administration -> Software Sources
In the ‘Download from’ box choose other.
And click on ‘Select best server’.
It will ping all the servers to check the connection speed with all of them and will select the fastest amongst them.
After choosing the fastest server, you will see that the download speeds rise significantly.