We did a zoom session during which I discovered that he was using two separate audio interfaces, one for input and one for output. Inserting that Triaz plugin caused the CPU to immediately jump to 10-15% even though it wasn’t even doing anything.
We switched to an aggregate interface (combining his separate interfaces into a single “virtual” interface) and that solved the problem completely. Inserting the Triaz now essentially had no impact on CPU and there were no more crackles with The Sub, etc.