https://bugs.winehq.org/show_bug.cgi?id=46056
--- Comment #2 from ajduck@outlook.com --- Just updating this issue to say it's still a problem, though I know this is more of a long-term/wider thing than a specific bug.
As I said, I think it may be more to do with Wine's multicore performance in general. I've found as of recently that setting both FL and wineserver to highest realtime priority seems to possibly improve performance in general (buffer underruns seem slightly less long) but I don't think it completely fixes what might be some sort of "Wine multicore/thread overhead" or otherwise potential for optimization.