Each arriving packet from wan, lan or wlan basically triggers the respective driver via IRQs, so high software IRQ load during speed test does not sound strange. CPU is not consumed for user-space computing, but is needed for network drivers (and firewall, routing etc.)
I just tested with flent using my 100/12 connection and I get
- 20% sirq load with wired
- 42%sirq load with 2.4GHz wifi
- 60% sirq load with 5GHz wifi
This is without any balancing the irqs to different CPU cores, as my "slow" connection does not usually cause any need for that.
There is useful IRQ balancing discussion in the R7800 exploration thread from early 2017. Read from here onward: Netgear R7800 exploration (IPQ8065, QCA9984) - #45 by Nague
But IRQ balancing is no magic wand. But may help in getting some additional throughput.
Ps. the script linked above by @philjohn looks a bit strange, as its actions do not match the comments inside the script. (both wan and wlan0 are assigned to the same core, despite the comment saying otherwise)