The owners of RT-AC5300 routers with latest firmware which contains the - Fixed CVE-2019-15126 (Kr00k) vulnerability is also causing high CPU usage for our models as well. Use the "Feedback" function of your router configuration page (under Advanced Settings-Administration) to submit the issue...
Same story here. I recently installed the RT-AC5300 router. Installed app on phone and one of the first things the app asked me to do was upgrade the firmware of the router to 81625 (3.0.0.4.384_81625). Within a couple days, high CPU on both cores. I SSH into the router and run 'top' to see...