Use ping, not ssh, to be able to have a more isolated test case and get quantitative data. In particular, separate loss from delay; both present as delay over TCP.
Run tcpdump on both ends, and compare timestamps of packets seen in both places, taking care to synchronize clocks (hard with network latency) or be aware of the mis-synchronization. Check to see if there are any power-saving settings for the adaptor and the DD-WRT box and diable them.
