There are many factors that may have attributed to slow internet speed, for example - the location, network traffic or phone/network settings.
If you are always experiencing slow internet connection, do run a speed test here and let us know the result. Information like - the location, timing and how did it usually happen (e.g. launching a certain app/ website) will help us greatly in assisting you.
Once you have the info, do contact our Customer Care Support here:-