Extremely slow disk write speed. What to do?

Question

Steam

The following issue occurred while downloading certain content in the Steam client.
First, the client works with data in some strange way, downloading and writing them to disk in chunks. Roughly speaking, I downloaded 5 megabytes, reduced the download speed, and wrote these 5 megabytes to disk.
Secondly, everything would be fine, but the speed of writing data to disk is simply negligible, which reduces the work with such a seemingly ridiculous amount of data into some kind of endless procedure.
All Steam settings regarding the download server are configured correctly, plus I tried to change the download region. There is more than enough free space on the disk that the client is working with.
The disk is defragmented, there are no errors or broken files.
The network is stable, the speed is high.
Antivirus is disabled.