OK, getting close.
I have found that the code running in the task on core 1 gets interrupted for 6.7us, every 10ms. It is the only task running on core 1 other than IDLE1 and ipc1. I don't know if it is every 10ms though. I assume that is the scheduler as 10ms is suspiciously similar to the tick time.
Unfortunately that is too long. Any ideas of how to get rid of that would be appreciated.
Thanks