When you add some rules into the firewall, it can be the case that Hardware Offloading is disabled without your knowledge! Before we re-enabled Hardware Offloading on the PPPoE client it was only able to handle about 140-150Mbps of throughput. After we re-enabled it, the throughput went up to 296Mbps on a 300Mbps BT Openreach FTTP circuit.
Try this from within the CLI to enable Hardware Offloading for the PPPoE Interface :
configure set system offload ipv4 pppoe enable commit save exit