[strangely I didn't receive Manuel's comment via email, but by checking
the gnats database.. Shouldn't it be sent on automatically?]

Anyway, bge(4) knows about vlans, so it doesn't have the "can't cope with
the extra 4 bytes so reduce mtu" problem. Both bge and vlan interfaces
are set to 1500.

Doing the above test on the same machine with a 3com 3c905cx-tx-m gets
4.6MB/s in both configurations, ie, it doesn't matter whether or not we
are tagging.

Funnily enough, I get the same result (4.6MB/s in both cases) for an old
realtek card on an ancient laptop. rtk doesn't understand vlans, so the
vlan interface has a lower mtu, but this didn't make the interface noticably
slower in my primitive test. We aren't talking fractions of second here!