I do not only think it is
When you benchmark the harddrive on the router it is capable of reaching ~40MB/s BUT programs like samba or transmission do other things than just write/read to/from the harddrive...
samba is quite a heavy protocol you know...
and transmission requires hashing (generating checksums) which is also a cpu intensive job
With my new RT-N16 which has a way faster cpu + more RAM things go a lot faster, up to around 8MB/s