Downloading any file and then suddenly a call comes on your mobile and after call, your download speed will increase.
Call Yourself, Then Decline it.
To increase internet speed reset your apn settings.
To implement this method you have to call the Airtel customer care service and tell them that you are getting a slow internet speed (if getting) and simultaneously tell your other friends to do this exact same thing.
Restarting device can increase internet speed but not only internet speed it will benefit smartphone's health.