Fast internet but very slow download speed.

Solution
Go into Task Manager -> More details -> Click NETWORK to filter the apps using the most network bandwidth.

There could be something eating up your connection without you knowing.

You should try using an ethernet cable temporarily to troubleshoot whether this happens.

If it does, you may need to reinstall a new driver for the Wi-Fi.
Go into Task Manager -> More details -> Click NETWORK to filter the apps using the most network bandwidth.

There could be something eating up your connection without you knowing.

You should try using an ethernet cable temporarily to troubleshoot whether this happens.

If it does, you may need to reinstall a new driver for the Wi-Fi.
 
Solution
I'd recommend the same thing as Asad.

In addition, ensure that the network driver is up to date. By hard-wiring, the computer with an ethernet cable will ensure you get the download speed you should be getting.
 
First thing you need to do is connect your PC with hardwired ethernet to router and test. I know it is inconvinent but this is a very needed step because it tells us if the problem is networking related or wifi related.

Also, is there other people/devices on your network? What speeds do they get?