What can slow down the download speed through the network adapter (Realtek PCie GBE Family Controller)?

Question

Realtek

Hello. For the first time in my experience, I encountered such a problem. My home network consists of 2 desktop computers, a router (Tenda W311R), and 3-4 phones are connected via Wi-Fi. The Internet is connected to the WAN port, the speed is 100 Mbps, computers are connected via network cables to LAN ports (standard crimping, comp-comp). The cables are intact, not damaged, the connectors also changed several times for the test. And that's the question, on win 7 computers, but on the first, the download speed is maximum (that is, about 90-95 Mbit), and on the second, immediately after installing Windows, the speed was the same, and after a few days, the download speed drops to 2-3 MB , despite the fact that the upload speed is maximum ( www.speedtest.net/my-result/3665150551) . Drivers tried different versions, sometimes it helps to demolish the device and install the driver again, but literally before the first reboot. Please help, suspicions fell on the integrated network adapter (Realtek PCie GBE Family Controller), maybe someone understands what could be the reason. OS stood a little, everywhere such problem. I don’t buy a new adapter, so you need to know for sure that the problem is in it, and not just guesswork. I can provide any additional information, screenshots, configs, etc. Help me please.

Answer 1

download speed drops to 2-3 MB

So megabyte or megabit?
Have you tried connecting this computer directly to the cable from the Internet bypassing the router?