It naturally happens. It is the bandwidth that determines the downloading speed. How many people are sharing the line is another factor (this is called contention ratio)
If you feel your broadband speed slows down, when you are downloading files, it is a good idea to upgrade your package to higher speed (8 Mb or higher).
What is the configuration of your PC? If it is old PC with low configuration, insufficient memory, that may also affect the broadband speed.
Check for the
high speed broadband packages and compare them to choose the suitable package for you.