I hate to say it but expecting reasonable performance through a USB 2.0 attached drive to a consumer router is a bit much. Routers are limited in both CPU and memory performance.
From a review of this
particular router they show the following speeds for reads / writes
Code:
FAT32 Write (MBytes/s) 12.5
FAT32 Read (MBytes/s) 13.3
NTFS Write (MBytes/s) 4.3
NTFS Read (MBytes/s) 12.0
shows that NTFS writes are relatively slow.