RPI3 + EXT4 usb drive on my side, forget the NTFS (check the CPU usage of the RPI when writing) if you can deal without (ext driver possible on windows)
To validate the RPI setup max speed can you try to D/L a linux distribution (ubunto, mint, etc have possible torrent download). This way we will check the max speed of your pi.
If not above the 500KB/s, check the network (wifi/lan?) and storage speed