[tproxy] squid+tproxy working with slow connection rate (plaintext)

[Auto]

I have squid 2.6.STABLE17 with tproxy compiled in, linux kernel (2.6.20.14) is properly patched with tproxy.
Tproxy is working fine and it's spoofing the ip addressed correctly, but there's very strange new connection limit.
When i run squid in transparent mode (without tproxy) i got 400-500 conn/sec with httperf and default parms, but with tproxy option i got about 9 conn/sec.
 
Here's the output:
httperf --client=0/1 --server=192.168.1.1 --port=65432 --uri=/ --send-buffer=4096 --recv-buffer=16384 --num-conns=1 --num-calls=1
Maximum connect burst length: 0
 
Total: connections 1 requests 1 replies 1 test-duration 0.110 s
 
Connection rate: 9.1 conn/s (110.2 ms/conn, <=1 concurrent connections)
Connection time [ms]: min 110.2 avg 110.2 max 110.2 median 110.5 stddev 0.0
Connection time [ms]: connect 1.8
Connection length [replies/conn]: 1.000
 
Even on localhost when i get about 4000 conn/sec i got only 9 with tproxy option.
Is there something that limits the number of new connections and delays SYN ?