updated to cope with the fact what Daniel Spreadbury claimed: Only WaveLab and Dorico uses Qt.
Even without retina, Cubase still has serious UI graphic performance problem with hi-resolution screens (especially in combination with certain VST plugins like the Voxengo EQ shipped with Cubase). This is likely due to the current cross-platform framework which is unable to utilize GPU for UI rendering.
If Cubase wants to fully support Retina mode under macOS, they must find and apply a new solution FIRST to completely replace their current plan. OTHERWISE IT WILL LEAD TO EXTREMELY SERIOUS UI RESPONSIVENESS ISSUES.
I bet this process could take at least 5 years for Cubase (because they have to completely rewrite and retest the UI part for Cubase). Note that this respects to the fact that Cubase constantly adding new features per major upgrades.
According to the moderator “MRJJ” at Qt official forum, “Qt Quick + QML instead + C++ backend” could be used in this case: https://forum.qt.io/topic/59578/rendering-performance-problems-on-imac-with-retina-5k/16