https://bugs.winehq.org/show_bug.cgi?id=46056
--- Comment #1 from ajduck@outlook.com --- Still happening as of Wine 4.11.
u-he's ACE plugin also slows down when that plugin's multicore option is enabled.
However, u-he's Repro-5 plugin does *not* slow down when it's multicore option is enabled and it improves performance as expected (almost halves processing time going by FL's new plugin performance monitor).
I looked at what was happening with the threads under htop.
Diva with multicore enabled used 15-16 (additional?) threads when 4 notes (4 voices per note) were held down playing 16 voices in total (a voice being one "instance"/"offshoot" of the synth). It seems that for every voice Diva and ACE uses a thread, and the maximum voices for both synths is 16 voices.
Meanwhile, Repro-5 seemed to only use at most 3-4 additional threads with multicore enabled even when holding down the maximum of 8 notes. This is with 1 voice per note as you can't stack voices with Repro-5, so 8 notes = 8 voices.
I say "additional" because with multicore options off there still seems to be mutithreading activity but under 3-4 common threads which probably has something to do with FL.
Anyway, perhaps the slowdown has something to do with some multithreading bottleneck as a lot more threads are being used in the case of Diva and ACE? Or perhaps Repro-5 is using some different Wine functions/codepath to Diva and ACE that avoids a slowdown regardless of how many threads used? This is just speculation/guessing of course.
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I also wonder if this issue could have some relation to why FL Studio in general slows down as a project file gets bigger. As more plugins are loaded and lots of stuff is showing in FL Studio's GUI and playlist (where everything gets laid out in a project file), processing speed can immensely slow down compared to what it would be on Windows, even when using FL features like "smart disable" (which disables a plugin after some time when no MIDI/audio input is going to it). More plugins = more threads, and FL's GUI is rendered entirely by the CPU (I may make a separate issue for this).