I had this issue and is now resolved...
1) Reset to factory defaults DIR-655
2) Reset DNS-323 to factory defaults as well
3) done.
This bumped me from 12MB/s to 24MB/s copying 800MB file in less than 30secs from my PC on second floor, wired down to basement, patche to DIR-655 in living room (approximately 100ft of CAT6 cabling). Jumbo frames doesnt help in small LAN's (got that from the God Mod's). Of course the DIR-655 handles up to 4000k jumbo frames...so if you want to prevent switching...set all to 4000k frames. Anything above is useless. In fact Jumbos are usless for small LAN's but if it makes you feel better set it up. In my experience I haven't noticed any improvements. If you are testing test several times to get an idea of what average throughput.
MOBO: Gigabyte--
Cheers!
Lucid