Tomato and super slow NAS speed?

So I just got an external drive to plug on to my Linksys E3200 router running with Tomato Firmware 1.28.0000 MIPSR2-3.3-138 K26 USB Mega-VPN. I managed to set it up just fine but when I tried to transfer some data on it I was shocked by the slow speed. If i plug the HDD on my PC i get around 20MB/s. If it is plugged on my router it is only 2-3MB/s.

I checked CPU/RAM load which is still very low during data transfer
HDD is formated with NTFS + GPT
Router is connected via Gigabit Lan

Are...

Tomato and super slow NAS speed?