Slow upload speeds on Windows 7

The reason could be many things. Let's start with the question, was the Ookla speed test server at the same distance as Dropbox's server? The answer to that would be maximum time no as speed test servers are hosted very near to customers in ISP network. They do not give real information like say you have 10Mbps connection, but will you really get 10Mbps once you cross your ISP's network? The answer is NO. No ISP will guarantee that once traffic leaves their network. So thats the connectivity part.

Secondly, it is latency that would be very harsh on data transfer speed. Even if you have 10Gbps connection, you can transfer at very low speeds because of how TCP works (a widely protocol used to transfer data). You can search these terms if you want to understand them in deep: "calculate tcp throughput with latency" or "tcp speed calculator according to delay".

Third, different speeds for different paths. I have seen this in the past many times. In your question, you have not mentioned your actual/bought/committed internet speed. I have seen ISPs who would grant you different speeds based on different destinations. Like, for Google/Youtube, they might have a dedicated connection with them and since they pay for that connection at a almost fixed rate every month, they want to utilise that connection as much as possible. So they provide uncapped/more speeds as compared to the committed speeds. I have seen connections with 10Mbps committed speeds getting 100Mbps on youtube to se videos and on microsoft to download windows/office updates.

So these were some of the reasons you can get different speedtest results. You cannot do apple to apple comparison between different destinations.

If you want to dig deeper into this, we can do that but before that, I would need few more information like whats your committed/bought internet speed, who is your ISP (My experience in networking domain helps me in knowing ISPs behaviour and how their service would be), etc.