After a bunch of scratching my head on why my speed tests weren't going great guns, I had a poke around google for others using their ERL as a WAN+2LAN + PPPOE client setup.
For some reason, it seems that hardware offloading is *off* by default when you use the wizard to setup the ERL. Turning it on made a huge difference in speed test results for me.
You can check whether it is on in your config tree, but if it isn't, this worked for me:
set system offload ipv4 pppoe enable
set system offload ipv4 vlan enable
set system offload ipsec enable
set system offload ipv4 forwarding enable