I have win 10 64-bit home on a 4th gen core i5 laptop. I have a WDC 1T 2.5" USB3 drive. It is about half full. It used to do 60-70MB/s write speed. Since a few days ago, I noticed it has slowed down by a factor of 5-6 to 10-20MB/s. While copying large files, I watched what's going go. The System and compressed memory process was doing a lot of read and write on the disk. If the copy speed reported by the dialog is 15MB/s, then system compressed memory process is reading 15MB/s and writing 30MB/s. It looks like this process is reading back the content written to the disk and writes back. It is consistent over reboot with several large files from 200MB to 7GB in size.
A couple of weeks ago I did a file backup. I created a 5GB virtual drive on my SSD and dumped all files to be backed up. Took only 5 minutes to copy the files to the virtual drive and copy the virtual drive file to the WDC disk, at around 67MB/s, typical of this type of portable drives. While I was wondering what was going on, I tested the same files with a USB3 dock and a 2TB Seagate drive. Write speed is about 150MB/s. There was no sign of the system and compressed memory process hogging R/W on the drive. So I thought it was not a problem with all disks. The WDC drive has no compress to save space option, BTW. I wonder what's going on. I have AVG antivirus and I turned it off while copying and same slow 15MB/s speed. All my drives are formatted into NTFS. My main drive is SSD and the source of the large files.