I just sent the following message to support, but thought it might be useful to check if some of you share this problem. I have noticed the strangest behavior when working at 96KHz with Pro Tools HDX. This is not happening at lower sampling rates (44.1/48 - didn't try at 88).
As a HDX user, I use both native and dsp plugins. The mixing necessarily happens in DSP domain. Please note that my Mac is a 12 core Mac Pro, almost the most powerful one can get from Apple.
Here is a way that, I hope, will allow to reproduce the problem :
1/ Create a 96KHz Pro Tools session with one single stereo track and import some audio (or not).
2/ Add reference 4 to a plugin slot leaving room above and below for testing
Open the System Usage window. On my computer, it shows around 8% on the global meter (the topmost one)
3/ Add a DSP plugin AFTER Reference 4. You should notice no change at all (8% cpu)
4/ Move that plugin ABOVE Reference 4. BOOM CPU goes crazy, 100% usage with one single native plugin on the most powerful Apple model. Add a few tracks or plugins and you cannot work, whatever the buffer settings. Changing Reference 4 from Phase Linear to Low Latency makes no change.
5/ Change the plugin from DSP to Native. Back to normal, 2% cpu usage.
It seems that, at high sampling rates, the dsp to native conversion has a big, big problem. I did not notice such a behaviour from other native plugins, even cpu intensive ones such as iZotope's Ozone (the full version).
This makes Reference 4 unusable at 96 KHz, any ideas ?
I'm attaching screen captures. I tried with several plugins. Of course, removing/disabling SW4 solves the issue.