I didn't read the posts before this. I skimmed a few.

So... the issue is related the Halion and involves midi tracks. I'm guessing the processing of the midi tracks in real time is the problem. (Duh...)

So in this case, why not simply FREEZE the offending synth tracks? Freezing lets the program write the synth track to audio and waa laa, it doesn't have to process in real time. you have the audio and the synth is off line for the playback. It's easy to unfreeze if you need to edit it. Other than that, you don't really need to have the midi processed in real time anyway. Freeze it and use the easy to process audio.

Kinda curious.... I believe you said there were multiple midi tracks..... have you tried running it with a single midi track to see if it can handle the real time processing without stuttering?

I used to work a lot with midi tracks and CPU intensive plugs and synths.... and on a barely capable laptop. I had 80% or better CPU duty cycle and the resultant pops, clicks, and even audio engine drops. The solution was to freeze the tracks and synths that were drawing the CPU power. I could freeze one or two of the main culprits and my CPU usage would drop from 80% or more to well under 40% and I could run problem free.

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www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.