A "Download Accelerator" it's a "client" thing, not a "server" thing. You don't have to install a "Download Accelerator" in your server, it's "who" downloads files from you, that should install it.
I'm 90% sure this it's a ISP issue. "ISPs often employ deep packet inspection (DPI)", so, your SpeedTests will always have awesome speeds, but then, when you use a "bandwidth intensive application, such as a server, [your ISP] might limit, or throttle, the rate at which it accepts data" (explanation taken from Wikipedia). For example, Verizon (and other ISP), often do this.
Your best bet, it's trying another application, and see the results. You may try another HTTP server, or any other program, like FileZilla (FTP). Try sharing files over FTP using FileZilla, and see if it's working better.
https://filezilla-project.org/
http://en.wikipedia.org/wiki/Bandwidth_throttling#Application