How are you connecting the HDDs? Not all USB ports may be USB 3.X
Plus the HDDs themsleves my be slower, which ones did you actually buy?
I used Win 11 and I do a TON of video encoding and transfer. I get the max transfer rates on my 2.5 GB ethernet from Win 11 to my NAS and NAS to Win 11.
On my local back up USB drives I also get the max data transfer rates from PC to HDD and the other way.
I connect with a 3.0 USB port.