As someone using ADC in Nuendo
, I was surprised at first to see how much CPU was used, but once I started thinking about how much buffering has to go on, it starts to make sense. What really hits the cpu is routing. Straight inserts aren't that bad, but once you start compensating sends and groups, things get pretty complex pretty quickly. Given that delay based/memory type operations have always been the weak spots for Digi hardware, I wouldn't be surprised if PDC isn't a cheap adventure.
This is most likely why they are offering the Accel upgrade to Mix and HD users. It may be that PDC makes a Mix system very anemic very quickly, and even an HD might be unhappy with the memory requirements (it might end up like the old days where you don't have enough of certain chip to do an operation.)