The newest DIR-655 firmware is 1.35NA. The router can't detect the newest firmware. A3 can use 1.32NA, 1.33NA, 1.34NA, and 1.35NA (can't downgrade with this version). FW 1.21 allows downgrade back to your original firmware if you have a copy of the original firmware. From 1.35NA back has to use 1.34WW or 1.31EU in order to downgrade back to 1.34NA/1.33NA.
Slow speed could be QoS, Connection Speed setting, and channel selection. Also has to enable the multicast streaming, the endpoint independent for the NAT, and the uplink speed.