Your Service Provider. Some service providers run faster than others. The biggest difference is between copper networks and fibre optic networks. On copper networks, your distance from the exchange is the largest factor. On fibre optic networks this is not an issue. Capacity in the local network also comes into play if too many people on your street are all trying to use the same infrastructure. On fibre optic networks this is less of an issue as the network has higher capacity.
The package you have purchased from your service provider. If you have bought a 10Mb package your will only get 10Mb, if you buy 100Mb you will get 100Mb. Sorry if that is stating the obvious.
The processing speed of your computer and router or modem. Old computers with less memory run slower.
The speed of the connection between your computer and modem or router – that is, a wired connection is faster than wireless. Older wireless connections run much slower than newer wireless connections. For maximum wireless speed you should look for 802.11n that can operate in the 5Ghz frequency range rather than the 2.4 GHz range
The time of day, your speed may slow down at certain times, for example, in the evenings when everyone is using BBC iPlayer
The speed of individual websites you are visiting (some have a maximum access speeds)
Whether you’re sharing your connection with other people or devices (like other laptops, smartphones, iPads or gaming consoles) in your home
Whether you have software running in the background, such as spyware