make sure there's nothing attached to the serial console if the device has one.
this was a very hard issue to track down: we had devices with long serial cables connected to the router and the other end of the cable (~10m long) wasn't connected to anything. it picked up some EM noise and constantly trickled the serial port thus the serial console.
serial console is an interrupted device in RouterOS, so it will bully the cpu all the time. do a /sys resource irq print to see what you have there:
[admin@hgw] > /sys resource irq print
Flags: ro - read-only
# IRQ USERS CPU ACTIVE-CPU COUNT
0 3 usb1 auto 0 65
1 4 switch0 auto 0 3 179 276 774
2 16 beeper auto 0 797
3 ro 19 serial auto 18
4 112 ts auto 0 0
the values next to serial should be low. if it is high and increasing rapidly, you have the same issue as we had.
then you just need to disconnect the cable...