My RB4011 has cores at 100% at less than 1 Gbps without FastTrack on v7 ...
I have the opposite experience: my hAP ac2 was at 15-20% under v6 when doing 30Mbps (at the time I was using 30/5 VDSL), the same unit now is at 10% when doing 980Mbps (I have FO 1Gbps/100Mbps) on v7. Alas: I did netinstall the hAP ac2 and I configured it from scratch ... with similar functionality (but not identical, I did take into account changed defaults when doing configuration anew). The proof that v7 does use fasttrack is that if I run speedtest against IPv6 host, CPU load on hAP ac2 goes above 50% while measured throughput drops to around 400Mbps.
Hell, even RB951G, running v7, was able to route at around 850Mbps (with slightly less than 100% CPU load) - I was using it while I was doing hAP ac2 config anew.
So I'd say that your config is somehow hosed due to frequent upgrades, downgrades and config changes ... and it's time to start anew from netinstall and new config (only use old one as a reminder of what functionality you need, but try to stick to default config philosophy, it's a good one unless you know much better.