I get tops 4 MB/s, and it's a USB 2.0 port. That's about the best you will get. The USB HDD feature is just a nice ad on, it was initially not even intended to be a feature on the DIR. Comparing this connection to direct USB connections is not a realistic comparison, not even for dedicated NAS devices. Only a $400+ device (Synology) wil give you 20 MB/s speed. And it does not have to do all the other things a router does.
The limitiation has to do with processing power and memory. Don't forget that the main feature of the router is not to suffer from this USB gadget. Translating USB-2-Ethernet and vv requires quite some processing power. So if you want NAS performance, only a dedicated NAS device with dedicated hardware will give that.