Hi,

AudioProcessorUsage() does not take the time needed for the DMA ISRs into account.
Some need quite a time (like S/PDIF emulation on Teensy 3.x).

I'd suggest a function like update_all() that informs the library about the needed time only.

foo(microseconds)

The time then can be added to the usage.
It is more or less constant and changes only when the CPU Speed changes, so there might be a more efficient way, too.
A way would be to simply use a good const estimate maybe something like "this output needs ~0.3% on Teensy 3.6", as does not needs to be that exact.