For all who are still facing this issue. The original thread was closed two years ago, so I need to start a new one.

There is an issue, that when a hard drive is connected to an USB port of the router the CPU usage is near 100% while transferring from or to the drive, yet speed doesn't go above 3Mbps. I've found myself with exact problem and I think I've found a solution. The drive needs to be formatted as FAT32. I believe the the issue is with routers NTFS drivers, and there's no sorry for exFAT.

To format a large drive as FAT32 you'll need some external tool, such as Rufus, since Windows doesn't support this FS for some large drives.

My transfers go as high as 60Mbps, yet CPU stays below 75%.

How that can help some of you.

Cheers

Rafał