Ok, then I'm presuming you should see the same speeds as before if you are just swapping one router for the other. Just glancing through the user manual, things to possibly check:
1. QOS page 28. Turn this off for now for test purposes.
2. Traffic Monitor. Page 31. Can this be turned off? Just wondering if the monitor function results in lower throughput. A similar effect has been observed with Merlin firmware. Turn the logging on, or other selected funtions and the throughput drops.
Other than that, I'm stumped. Need an AC68 expert to chime in here with further ideas.