Poor NAT throughput on Atom D525 CPU

I had a Atom D525 CPU with 6 * Intel 82583V chip loaded with RouterOS
My ISP use VLAN10 for internet traffice and VLAN20 for IPTV, so i setup these VLAN on ETH1.
I setup Bridge1 to bridge VLAN20 and ETH6 for IPTV.
I also setup Bridge2 for ETH2-ETH5, and assign DHCP server to it.

I managed to go internet and watch IPTV, but the speedtest performance is poor. I can only achieve 200+Mbps at CPU usage 15% while i subscribe to 500Mbps plan.
When i change back to the router provided by the ISP, i could get 500+Mbps, so i am not sure which part of my setup goes wrong.

Do i have to setup QoS for the 2 VLANs? How can i do it? I do not know much on scripting, i only configure the RouterOS with Winbox.