I know the speed would reduce when it is going through the router's USB port, but I'm not sure what is the speed it can be:(
It is said that only 2 users can access the FTP sever at the same time.

I had a play with the USB ftp function and used filezilla as the client. Uploading from a client to the drive wirelessly gave about 1.2MB/s on average and downloading from the drive (a quite old USB 2.0 freebie) about 3.4MB/s - I don't tend to see >10MB/s for any USB (USB 2.0) transfer so after that it depends on the device.

Connecting the same USB drive directly to my laptop gave me approx 10.8MB/s "download" and 890KB/s climbing over the transfer to 1.8MB/s "upload".

I would test your USB drive speed directly in a port attached to your PC - 200KB/s does seem slow - was this a single large file or many small files? There is a lot of overhead associated with copying small files over a network (I understand) such that it is sometimes recommended to package them in a single archive when sending a large number this way.

I've since set up an FTP server on an old netbook connected through one of the LAN ports - gets about 7-11MB/s upload on average - that's to a spinning hard-drive with some cache to it of course.

These are normal speeds for usb:1-2 MB writing and 4-6 for reading.

Hello,
I have tried to DL torrent file to the USB HDD attached to USB of the router. The max speed was 200 kb/sec. After this tried to copy file from PC to USB HDD on router. Max speed was 1,2 MB/sec.
Why there is so low speed? USB is not usable in this case.

Region : Bulgaria

Model : TL-WR842ND

Hardware Version : WR842ND v2 00000000

Firmware Version : 3.14.2 Build 130628 Rel.37450n