Download speed gradually slows down

Maybe your SSD is DRAM less. Try with other system or to another SSD with DRAM or NVMe.

Even though a good HDD gives you full 1 gbps constant.
I have two Enterprise HDD with 7200 RPM of 6 TB and 16 TB, and both gives me perfect 1 Gbps.
Getting proper 1 gbps speed completely depends on your hardware as well.
 
Pinpointing the causes of slowdown is difficult.
  1. I would start solving it with proper UEFI Setting. Example : Disable the CSM.
  2. Then I would go for installation of all the necessary system drivers.
  3. I would install a stable WiFi Network adapter driver.
  4. The next could tuning the Wi-Fi Router settings.
You may check the download speed by connecting the computer to router using LAN cable.
 
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