That TCP result's maybe due to your setup, because my RB5009 as PPPoE client on GPON has no problem running speedtest.net with results near 2.3Gbps (IPv6 speedtest number, the with overhead the throughput on the ethernet interfaces is of course higher, a little bit over 2.4Gbps if my calculation is correct, and that's near the GPON download limit). Firewall config is defconf + addititional rules.
And fasttrack is not needed to reach that result, here is my old post from last year when there was no fasttrack support for IPv6 RB5009 and 2Gb/s internet speed - #20 by CGGXANNX. With fasttrack the CPU load is of course lower (old post).