Whee... Another distraction.
When I have an nEDBG eval board (Curiosity Nano) connected to my Mac (10.13.6), and have it's virtual serial port open (Arduino Serial Monitor or minicom),, the "kernel_task" process kicks up to 80% CPU utilization !
This happens with Curiosity ATtiny3217 nano, Curiosity ATmega4809 nano, and Curiosity AVR128da48 nano.
They all have
Application version...........1.21.37 (0x01.0x15.0x25) Tool pack version .............1.5.210
It does NOT happen with Xplained Nano ATtiny416 (mEDBG) or Xplained Pro (full EDBG)
It does not seem to have a similar problem on PC.