Conundrum: shiny new PC acts as a much slower vlan box than an ancient one ?!

I have a working 1Ghz P3 with
ex0 at pci1 dev 12 function 0: 3Com 3c905C-TX 10/100 Ethernet with mngmt (rev. 0x78)
bmtphy0 at ex0 phy 24: Broadcom 3c905C internal PHY, rev. 7

running ipf 4.1.6 with a tiny number of rules, and with eg

ex0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        capabilities=7<IP4CSUM,TCP4CSUM,UDP4CSUM>
        enabled=7<IP4CSUM,TCP4CSUM,UDP4CSUM>
        address: 00:b0:d0:ed:3a:a9
        media: Ethernet autoselect (100baseTX full-duplex)
        status: active
        inet6 fe80::2b0:d0ff:feed:3aa9%ex0 prefixlen 64 scopeid 0x1
vlan1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        vlan: 1215 parent: ex0
        address: 00:b0:d0:ed:3a:a9
        inet 192.168.80.62 netmask 0xfffff000 broadcast 192.168.95.255
        inet6 fe80::2b0:d0ff:feed:3aa9%vlan1 prefixlen 64 scopeid 0x4
vlan2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        vlan: 692 parent: ex0
        address: 00:b0:d0:ed:3a:a9
        inet 192.168.0.62 netmask 0xfffff000 broadcast 192.168.15.255
        inet6 fe80::2b0:d0ff:feed:3aa9%vlan2 prefixlen 64 scopeid 0x5

plugged into a 3COM 3300 which agrees with the 100 FD setting.

It is rather on the slow side copying 600MB through the vlans in 4 hours
(about 50kb/s). So, I thought we could just throw some hardware at the
problem and replaced it with a 2.8GHz P4 with 800MHz FSB and

bge0 at pci2 dev 0 function 0: Broadcom BCM5751 Gigabit Ethernet
bge0: ASIC BCM5750 A1 (0x4001), Ethernet address 00:11:43:7c:6c:3a
brgphy0 at bge0 phy 1: BCM5750 1000BASE-T media interface, rev. 0

bge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        capabilities=7<IP4CSUM,TCP4CSUM,UDP4CSUM>
        enabled=7<IP4CSUM,TCP4CSUM,UDP4CSUM>
        address: 00:11:43:7c:6c:3a
        media: Ethernet autoselect (100baseTX full-duplex,flowcontrol,rxpause,txpause)
        status: active
        inet6 fe80::211:43ff:fe7c:6c3a%bge0 prefixlen 64 scopeid 0x1
vlan1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        capabilities=7<IP4CSUM,TCP4CSUM,UDP4CSUM>
        enabled=0
        vlan: 1215 parent: bge0
        address: 00:11:43:7c:6c:3a
        inet 192.168.80.62 netmask 0xfffff000 broadcast 192.168.95.255
        inet6 fe80::211:43ff:fe7c:6c3a%vlan1 prefixlen 64 scopeid 0x8

To my surprise, this is much slower(!) (same switch port etc, 1 days -current
difference - 3 March vs 4 March, so both have sack and same ipf)

Any idea where to look further? (vlan and bge seem to have a checkered
history..)

Cheers,

Patrick