It's with PPPoE on the RB5009, but the RB5009 also acts as normal router, not the setup from the post mentioned above. I only have WAN through PPPoE. Like I wrote, 2.4 Gbps is probably not the real limit because the router is only at 70% CPU load without fasttrack when doing that. And here is an older post with the screenshot with the CPU load WITH fasttrack:
You can see that TX and RX on ether1 and sfp-sfpplus1 (where the GPON stick is plugged in) reached 2.4 Gbps while CPU usage is below 50% with fasttrack. So the limit is from my ISP and GPON. GPON also limits the upload at 1.2Gbps and my ISP only allows 1 Gbps upload.
As for the config: It actually just the defconf as starting point, I even used QuickSet when it first popped up to switch from DHCP to PPPoE. Since then I've added more rules to the defconf firewall (but the accept established / related rule that handle most packets when fasttrack is not active is at the same position anyway, and the added rules are below it so have no big effect).
That setup achieved the speed above. I've since made it even more complicated by moving all ports into the bridge, even the GPON port and use VLAN-only. The PPPoE now sits over a VLAN interface under the bridge, that has the sfp-sfpplus1 port as access port, so in theory even more overhead. The screenshot above is from this setup.
Also, for a long time RouterOS had no fasttrack for IPv6 and speedtests done with IPv6 have no problem reaching that load. The reported throughput in the speedtest app is a bit lower (a few tens Mbps lower) due to the IPv6 header overhead, but the rates on the ethernet ports are the same. Now RouterOS has fasttrack for IPv6 too and it works well.
So PPPoE does not overload the RB5009 at 2.5 Gbps. However, looking at the linked post. If you applied that setup with mark-routing then you'll need to disable fasttrack. So the CPU load will probably raise to 70% for 2.5 Gbps.