If the router stats are showing proper downstream rate, what may be the cause for slow download speed?

Run speedtest.net and see how your local connection looks.

Download WinMTR and run against the site you are downloading slow from. If you see slow hops or drops, this will give you some actionable data.

Nothing identified, the problem may be that your system is busy. Hit<ctrl><alt><delete>, Task Manager and see what is running on your system, how busy it is.

You have absolutely NO control over the rate of the downloading server. The downloading server is the server at the remote site that feeds the download.

In many cases, the downloading server is limited to a certain max speed and as more and more users log on to download, the speed drops below the max speed.

Further, many downloading sites have a bank of downloading servers and some are faster, others are slower.

If you are assigned to a slower one this time and were assigned to a faster one previously, you may think the issue is on your end but it is not.

all the above answers are confusing, or just wrong.

let's make this simple.

You have a connection to the internet, where you are downloading from also does.

if you are at max throughput, then the other end must be the problem.

you are always limited by the smallest pipe. In this case, where you are downloading from is the cause.

You can use speedtest.net to see what your download/upload speed is. Once it's determined it's not your ISP, it must be the other end point. There's zero you can do about it.

There are two options for this. The server from the site you are downloading from, or your processor speed.