Doing some testing today, disabled SQM, and turned on software and hardware flow offloading.
Archer C7 V2, on a 50/50 fiber connection.
I was surprised at the results...
htop showed the CPU didn't go over 19%
There is no hardware flow-offloading on ath79 yet (maybe never), so at best that setting doesn't do anything. While software flow-offloading should work on all hardware, I'm not so positive if your observed CPU usage really indicates it being at work - the c7-v2 should be able to do around (just under) 200 MBit/s routing without any offloading in place (100% CPU usage, and choppy under that kind of load), at 50 MBit/s 19% CPU usage shouldn't be that far away from the expected values (~25%, more or less).
On the plus side, your router is too fast for your WAN speed 
Why I put the question mark at the end...
Not for long...
Speed tier jump sometime this year.