I'm running the russian firmware
https://code.google.com/p/rt-n56u/
20-30MB/s seems reasonable, even on my USB3 SSD I could only get about 40MB/s read from a regular 7200RPM HDD. But right now, my transfer rate is hardly at 20-30MB/s, it will start off around 20MB/s then slow down to around 10MB/s, before disabling the LSO, the speed was even worst than my WAN speed! It was faster to re-download something off the internet than to copy it via the internal network