Did you install, uninstall, change or otherwise modify ANYTHING on the system whether it's hardware, software or settings, on either the PC, router or modem, at or around roughly the time on Friday that you began noticing that there was a change in speed?
Have you checked again since then?
The fact that your network usage shows only 1% probably doesn't mean much, as that is likely referencing 1% of your network adapters capability, not 1% of your actual ISP tier maximum bandwidth. And it also very much depends on what kind of connection you have. DSL connections for example are generally just you between your house and the local central office, not shared. Cable connections are generally shared with other users in that neighborhood or section of the network. Fiber optic, generally also shared but the bandwith is so high it typically doesn't matter, just what tier you're on.
I get like 375-425mbps speeds when I run speed tests as well, but in real world applications I never see those kinds of speeds because it's dependent on how far away the server you are trying to access is, how many others are accessing it at the time, how many others are accessing OTHER sites that share the use of the networks BETWEEN you and that server even if they aren't actually accessing the same server, the time of day it is (Because different times of day will generally mean more or less people online at that time) and also sometimes if some normally used section of the network is down and traffic has to be re-routed through other, longer means of "getting you there" to where you are trying to access.
Bill knows far more than I about this so whatever he says likely greatly outweighs what I can offer, but so far as I know all of THIS ^^^^^ is completely accurate. Which still doesn't necessarily mean that any of those things ARE your problem.
Another thing to consider is even if you uninstall a program that has made changes to your network configuration to supposedly "optimize" it, it doesn't mean that it always changes those settings back. Sometimes it's necessary to reinstall the network adapter drivers or even in some cases completely reinstall Windows if you don't want to go through a lengthy process of finding the unwanted configuration setting somewhere in the registry or device configuration settings.
Probably not a bad idea to go to the motherboard product support page (Or the page for your NIC if you have an add in card installed) and download/install the latest driver, even if it's the same one you already have, so that the settings are returned to the default configuration.