The RB2011 is "rated" at about 700Mbit/s: http://routerboard.com/RB2011UiAS-IN
(the value in the lower right corner is closest to real-world) so it is unlikely that you will get 1 Gbit out of it anyway.
Please post a configuration export.
To test for a bottleneck first clear
the router configuration:
sys reset-configuration skip-backup=yes no-defaults=yes
Then configure the router as a simple switch:
set ether1 comment="WAN"
set ether2 master-port=ether1 comment="LAN"
That should behave the same as connecting your PC directly to the modem.
After testing reset the router again and configure it as a software switch:
/int bri po
add int=ether1 bridge=bri1
add int=ether2 bridge=bri1
That should still behave the same way as above (but with higher CPU load).