What effects download speed?

Answer 1

Primarily the speed of your internet connection and the contention it may have on it.

However, 'malware' (viruses and spyware) on your computer may also slow it down, as might a slow processor and too little memory.

Answer 2

It's not so much as what data affects the speed but how much. The more data the slower the computer response, even if it's just for a moment. Data that's constantly transferred back and forth as if one computer talking to another regularly then it can affect the response on a computer. Programs running in the background do the same thing but it's still data being processed. Prime example is virus protection programs are always doing checks on the computer and internet traffic. This will slow down the effectiveness of the computer but in actuallity the speed is still the same. You just can't see it because of all the activity going on. Some refer to this as a funnel effect.