RB450G - what throughput speed is normal?

RouterOS Beginner Basics
1

I want to use RB450G as border router to Internet for three-branched network.
My WAN link is ethernet 50 Mbits/sec, and now working on linux gateway (Pentium4+Fedora 10).
When I try to change linux gateway with RB450G - the loading of WAN link decreases to 14-15 mbit/s (with static NAT on WAN port).
I checked RB450g “on a table” with two computers, and I see that in the elementary configuration as a static router without any NAT and other services throughput speed a maximum approximately 45-60 mbits/s.
Now I have two question:

  1. What for the RB450G has gigabit ports onboard? After all it is enough 100-mbit.
  2. If it possible, how to load WAN port at its full capacity (50 mbit)? maybe RB450G isn’t right choise for this task?
2

Well im new to RB450G
But my personal tests give (on a 100/10 line)
90/11 mbits (With nat)

On solid cat6 lan 600+ mbit. (tcp/ip)

So something should be wrong with your RB. check the are you sure your not using bad networkcards for the tests ? else your RB should be busted