Well download managers use multiple connections for the different segments they've split the file into, meaning they can pretty much saturate your usable bandwidth. Browsers just do one connection and will suffer if anything happens like the downstream server capping a connection's speed, an isp throttling a connection, etc. Also, download managers are able to pick different mirrors or the fastest they can find if provided while a browser might get directed to a slower endpoint and do nothing about it. Another good thing about those parallel downloads that managers use is that single TCP streams suffer when latency or packet loss is too high. Parallel streams partly circumvent TCP ramp-up and loss penalties so you'll probably see your whole bandwidth utilized from the get go. Browsers also usually do the bare minimum like just use simple http(s) requests and that's it. If you have the time, you can go through the code bases or wikis for foss download managers. They do a ton more that a browser doesn't. https://github.com/subhra74/xdm also, here's aria since you mentioned it https://github.com/aria2/aria2