Incredibly Slow Internet on Only One device

[LazarAFK]

For the past few days Internet speed on my PC has been terrible. It takes a minute to load 1 website. Downloading/updating any steam games is impossible.
I am confused as to why this is happening.

Here's my setup: There's a cable going from the main router to my router (so I can have good WiFi in my room) and there's another cable going from my router to my PC. This setup worked fine for a year until a few days ago.
My PC is the only device that has slow Internet. Every other device that's connected to either of the routers works fine at full speeds.
When I do a speed test it shows me the full download speed but that's obviously not true.
I'm scanning my PC for viruses atm, but is there anything else I can do?

 

[LazarAFK]

One thing I forgot to mention is that some things like Youtube videos work perfectly (although loading the youtube website is still very slow).

 

[Ralston18]

Windows 10? - Run the built-in troubleshooters. They may find and fix something.

Failing that, use Task Manager, Performance Monitor, and Resource Monitor to observe what your PC is doing at any given time.

Open one the three tools after booting and watch. If all seems well, slide the window to the side and proceed as you normally do. Watch for changes that occur when the slowness begins.

Look for some app, process, or service that is consuming all or most of some system resource.

May take you some time and effort to get a sense of things. No rush and be careful not to jump to conclusions....