Thanks for all the answers, but the problem still exists.

To sum up:

OpenBSD 4.8 default install

cpu0: Intel(R) Celeron(R) CPU 2.80GHz ("GenuineIntel" 686-class) 2.80 GHz
npx0 at isa0 port 0xf0/16: reported by CPUID; using exception 16
em0 at pci1 dev 0 function 0 "Intel PRO/1000 PT (82572EI)" rev 0x06: 
apic 1 int 16 (irq 5), address XX:XX:XX:XX:XX:XX

1.
- tcp send and receive window @ default
- pf disabled
- transfer speed on em0 tested by iperf: 458 Mbits/sec
- top shows:

load averages:  0.90,  0.42,  0.18
25 processes:  1 running, 23 idle, 1 on processor
CPU states:  0.6% user,  0.0% nice, 51.5% system, 33.5% interrupt, 14.4% 
idle
Memory: Real: 9164K/43M act/tot  Free: 442M  Swap: 0K/759M used/tot


2.
- tcp send and receive window: 131072
- pf disabled
- transfer speed on em0 tested by iperf: 767 Mbits/sec
- top shows:

load averages:  0.85,  0.56,  0.29
25 processes:  1 running, 23 idle, 1 on processor
CPU states:  1.6% user,  0.0% nice, 71.5% system, 26.9% interrupt,  0.0% 
idle
Memory: Real: 9172K/43M act/tot  Free: 442M  Swap: 0K/759M used/tot


3.
- tcp send and receive window: 131072
- pf enabled, no queue
- transfer speed on em0 tested by iperf: 677 Mbits/sec
- top shows:

load averages:  0.84,  0.58,  0.38
25 processes:  1 running, 23 idle, 1 on processor
CPU states:  1.4% user,  0.0% nice, 70.1% system, 28.5% interrupt,  0.0% 
idle
Memory: Real: 9184K/44M act/tot  Free: 441M  Swap: 0K/759M used/tot


4.
- tcp send and receive window: 131072
- pf enabled
- to default pf.conf added (as Joel Sing suggested qlimit changed from 
50 to 500):
altq on em0 cbq bandwidth 1Gb qlimit 500 queue { q_lan }
queue q_lan bandwidth 950Mb qlimit 500 cbq (default)
- transfer speed on em0 tested by iperf: 337 Mbits/sec
- top shows:

load averages:  0.94,  0.68,  0.45
25 processes:  1 running, 23 idle, 1 on processor
CPU states:  0.6% user,  0.0% nice, 79.4% system, 20.0% interrupt,  0.0% 
idle
Memory: Real: 9184K/44M act/tot  Free: 441M  Swap: 0K/759M used/tot


root@router-test (/root)# systat queue
2 users    Load 0.69 0.58 0.48                      Wed Nov 17 21:34:38 2010

QUEUE               BW SCH  PR  PKTS BYTES DROP_P DROP_B QLEN BORR SUSP 
P/S  B/S
root_em0         1000M cbq   0 1803K 2666M      0      0    0    0    0 
293  44M
  q_lan            950M cbq     1803K 2666M      0      0    0    0    0 
293  44M


root@router-test (/root)# pfctl -vvs queue (transfer starts slowly, then 
at best time transfer is 322.11Mb/s than drops - i dont know is it a 
proper behaviour)

queue root_em0 on em0 bandwidth 1Gb priority 0 qlimit 500 cbq( wrr root 
) {q_lan}
   [ pkts:    4372637  bytes: 6619141152  dropped pkts:      0 bytes: 
    0 ]
   [ qlength:   0/500  borrows:      0  suspends:      0 ]
   [ measured: 26598.4 packets/s, 322.11Mb/s ]
queue  q_lan on em0 bandwidth 950Mb qlimit 500 cbq( default )
   [ pkts:    4372637  bytes: 6619141152  dropped pkts:      0 bytes: 
    0 ]
   [ qlength:   0/500  borrows:      0  suspends:      0 ]
   [ measured: 26598.4 packets/s, 322.11Mb/s ]



best regards,
RLW