How quickly the Adobe servers can send the data (usually pretty fast).
The maximum download speed of the Internet service you use. How many megabits per second is the download speed of your Internet connection?
Any network congestion factors in the middle that screw things up. For example, if an upgrade is just released, downloading can be slower if everyone is trying to download it at the same time.
Any network issues within your local network, like being too far from your wifi access point.
On a normal day you should be able to download the application as fast as your own Internet connection will allow. Your computer is probably not the problem — all recent computers, smartphones, tablets can handle very fast network connections. Any slowdowns are usually network-related.
If you do an Internet speed test (like this one), what is the download speed you get?