RB 750 bandwidth per ethernet port.

Hello to all. I want to ask a something about the rb750. The rb750 have 5 ethernet ports that can be routed separately. So i have 3 different networks.
10.0.0.0/24 ether2(hardwired)
192.168.1.0/24 ether3(wireless)
10.1.0.0/24 ether4(hardwired)

I assume that each port can provide 100mbit network. But when i transfer files from my samba server at the same time from the two networks, I find out that the speed is 100mbit for all ports. Can someone explain to me how can i get 100mbit for each ethernet port? (Do i need a gigabit routerboard?)

Thank you!

Throughput results
RB7xx/OmniTIK Series @400Mhz (Ethernet only test) 64 byte frames 512 byte frames 1518 byte frames
IP Firewall Conntrack Mode Mbps Fps Mbps Fps Mbps Fps
off off Bridging 47.1 92000 358.4 87500 452.97 37300
on off Routing 37.68 73600 294.91 72000 452.97 37300
on off Bridging 35.48 69300 230.2 56200 446.9 36800
on on Routing 28.42 55500 222.41 54300 445.68 36700
on on Bridging 18.94 37000 149.5 36500 425.04 35000

Notes:
All throughput tests done with Xena Networks specialized test system and RouterOS v5, according RFC2544, with Ethernet frame sizes 64, 512, 1518 bytes.
Each board is tested with specified number of Ethernet interfaces, to ensure optimal load on hardware.
Values in Italics indicate that the maximum possible throughput was reached, limited only by ethernet ports themselves.