Windows 10: Sudden Sluggish Internet Speed

Hi, Here's my problem. Over the course of several months, I've noticed a drop in my internet responsiveness. I finally got around to diving into the problem. My first thought was that I picked up a virus. I ran scans with Malware Hunter, Malwarebytes, Spybot Search & Destroy, Norton Antivirus, and Norton Power Eraser, but nothing came up except one or two tracking cookies. I'm running with Verizon FIOS Gigabit internet, so I ran the Verizon Speed Test, which they recommend. The results I got was 439 Mbps Down/760 Mbps Up from my router to their server

:)

 

Sluggish WiFi Speeds on Desktop

Unfortunately, the steps in the link didn't work.

• I have a TP-Link Archer C8
• I have Norton Antivirus; I tried disabling both the firewall and anti virus and no luck. My wife's laptop has the same version of Anti virus and no noticable difference in her Internet speeds. Hers also runs Windows 10.
• I've tried multiple browsers and various video games, but they're all slow. Speed Test is also saying i'm hovering betrween 4 - 9 mbs.
• Everything's been cleared out.

The USB i have is an Etekcity AC1200 USB

I can provide any information you need;,

 

Malware, Reset PC, Sudden Major Sluggish Speeds

Here's a first step:

Take a look in Task Manager when it's sluggish.

Look at the Processes tab.

What is using up the most CPU, Memory, and Disk when you sort by those columns?

What items show as "enabled" in the startup tab?

 

Sudden Downgrading of Internet Speeds

Since last Thursday (29th October) my desktop computer internet speeds have dropped to 10% or less than previous. Normally I have been getting the following (non fibre broadband):

ping 25 -80 ms; Download speed 17 +/- Mb/s; Upload speed 0.87 Mb/s

The speeds have dropped to:

ping 165 - 1896 ms; Download speeds <2.0 Mb/s; Upload speed 0.0 - 0.02 Mb/s

I have checked out my router (ISP provided), my ethernet cable, and my wi-fi internet connection. All work as previously when connected to my laptop (Dell Latitude with Intel Core i7-6600U @2.60 GHz 2.80 GHz) giving me the higher settings listed above.
I have replaced the ethernet cable with a brand-new cable - to no effect. I re-installed the drivers to the ethernet hardware - to no effect. I have added a USB wi-fi connector to my desktop (thus by-passing the ethernet input socket), again to no effect.
I would appreciate any constructive suggestions.

Something, somewhere (hardware, software or setting?), is "choking" my internet input into my desktop computer. My desktop specs are:

Intel i9900K @ 3.60 GHz, nVidia RTX 2080i (11 GB DDR6 RAM onboard), 32 GB DDR4RAM, 2TB HDD, 1TB SSD, 500 GB SSD, ASUSTek Z390-Plus Mobo (LGA1151);