Have you tweaked the CPU frequency scaling governor settings at all??
If not, SSH into the router and execute the following 2 commands:
echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
echo performance > /sys/devices/system/cpu/cpufreq/policy1/scaling_governor
Then re-run your tests.
These commands temporarily force both CPU cores to run at maximum frequency. The changes will be lost on a reboot, or can be reverted by executing the commands again with "ondemand" instead of "performance". See the R7800 performance and Netgear R7800 exploration (IPQ8065, QCA9984) threads for more discussion about R7800 performance tuning and making such changes more permanent.