I see there is now a new handy feature in the status page showing per-port status (negotiated speed and transfer totals). However, when transferring files between my NAS and PC there is no sign those per-port totals are changing, nor do the real-time graphs represent traffic speeds on the internal switch. However, running a speed test over my WAN to a public server is reported OK.
(so far not tested this on any other hardware though)
TL;DR it seems the router switch's inter-port traffic is not seen by any of the statistics tools.
That would seem to be the case, but I know the WRT3200ACM suffered from this bug:
Suggesting the firmware (or at least kernel module) is somehow in control of routing between ports on the hardware switch, so I wondered in that case if it could see the packet counters. But obviously I don't know enough about the switch/DSA/etc details to help in any way.