I finally did it, Aaron you were right. I've added seperate rules for incoming and outgoing traffic (using queues with 100% weight) :
If Proto Source Destination Target DescriptionWAN --> * * DMZ net 2000 Kbp/s queue
WAN --> * * VLAN10 net 48 Kbp/s queue
WAN <-- * DMZ net * 2000 Kbp/s queue
WAN <-- * VLAN10 net * 48 Kbp/s queue
Case solved.