Mysterious internet speed issues

I have fresh 6 months old self-built PC. I use the ethernet cable connection on my PC and other devices are connected on Wi-Fi to the same router. Normal internet speed is up to 72Mbit/s. I have internet speed issues since earlier today. Here are some interesting facts:

Download speed measured (approximates) on my PC ethernet connection:
- Ookla speed test: 4.53 Mbit/s
- Google Speed Test: 69Mbit/s (no issues there)
- Blizzard Call of Duty game download: 68Mbit/s (no issues there)
- GitBash application download: less than 100Kb/s
- Github repository 70Kb/s

On my Laptop Ethernet connection via USB adapter (Same router):
- Ookla speed test: 69Mbit/s
- Google Speed Test: 69Mbit/s (no issues there)
- Blizzard Call of Duty game download: 68Mbit/s (no issues there)
- GitBash application download: less than 3 MBit/s (Usual max speed)
- Github repository 3 MBit/s (Usual max speed)

On my PC Ethernet connection via USB adapter (Literally plugged the same cable, the only difference is that it's running via USB adapter into my PC):
- No issues there, all speeds at maximum as in the previous test

Ideally, I'd prefer it to work fast without the need for a USB adapter. And if there are any issues with my Network card, I'd like to fix them. I ruled out ISP and WiFi router, as it works fine on other devices. So far what I did:
- Tried to look for Network Card updates in Device Manager - Nothing found
- Reset network settings

Nothing worked so far. What shall I try next?

Also, Ookla speed test has some weird speed pattern, going fast at first and then slowing down, and it's the same every time I run the test:

Sorry, my bad
OS: Windows 10
Version 1909 (OS Build 18363.1256) 64Bit
not sure which desktop environment, but I can find out if you tell me how
It started happening earlier this afternoon since I stretched the Ethernet cable (a tiny bit, while moving my desk) and plugged a second monitor, not sure if it can affect it
I tried on Edge browser - the result is the same

I just updated to the newest 20H2. The problem is still there. Although I noticed Ookla detects different speeds when I maneuver the cable, sometimes it's 2Mbit/s, sometimes it's 20. I'll try to swap the cable when I get a chance. Although I find it quite strange that when I measure the speed using Google speed test there is no issue.

The USB cable that you are talking about is that a normal USB or USB and Ethernet connector.

Click to expand...

It's an Ethernet to USB adapter. I need it sometimes as my laptop doesn't have Ethernet socket

One thing I forgot to mention is that the Ethernet connector locking clip snapped which is probably the cause. I didn't think it was an issue as the speed is generally fine using a USB adapter. Will report back when I replace the connector