Well that explains the crazy slow performance on an old windows forms app I had written back when I was a junior developer. I'll try to see if I can reach anyone from my first employer and make them disable the debug output. Would be an interesting contribution, considering that I left more than 10 years ago :)
Old old Windows Forms had its own text rendering (and still has), but most controls now have a second code path that uses the system's text renderer, which got updated with better shaping, more scripts, etc., while GDI+ basically never got any updates. You can see that when there's a call to SetCompatibleTextRendering(false) in the code somewhere; then it's using GDI instead of GDI+.
