Or, unless they've changed it, hardware accelerated rendering. Winforms was based on System.Drawing, which used GDI+, which was largely software rendering. This was confusing because GDI+ was not really related to GDI, which had and still does retain some hardware acceleration support. Even basic color fills start becoming an issue with a big window/monitor.

Winforms is also .NET based, so it's inaccessible if you don't want to write your UI in and take a dependency on .NET.