running iperf test and bandwidth is low

hi all,

im running an iperf test between hosts on different vlans and the fw is the gateway between the hosts, there both 10gig connected and im only getting speeds of 4gig

where as when i run an iperf test on hosts on the same subnet i get speeds of 9.8gig

is there anywhere on the fw i can check to see why this is

EDIT - i turned off IPS protection as i thought it was that but made no difference

thanks,

rob