https://bugs.winehq.org/show_bug.cgi?id=56921
--- Comment #4 from Victini v_winebugs@outlook.com --- Had moments of lag again, so I went to check if WINE was actually using the native gdiplus, and indeed it was. I had set gdiplus to builtin and the lag disappeared, though elements that do use gdiplus aren't smoothed (or blurred in some cases) and look ugly as a result.
Switching my MusicBee prefix to use native gdiplus restores the lag, so it may be an issue relating to native gdiplus.