In Haiku windowing system, each app window gets its own thread so dialog boxes run in a different thread to the main window and a different thread to the core app. In Linux, all windows share the same message loop thread. A simple port reveals threading issues in Haiku which dont exist on Linux.

To work around this, all window messages in ported apps are marshalled to execute sequentially. Small additional overhead, and the system doesnt spread available threads, so noticably slower.

Compare a native Haiku app with a ported app, one is smooth as ice while the other isnt. Users notice it. This is on many core systems.