That will be our first port of call when investigating, the Qt rewrite :) It will definitely speed things up overall, whether it resolves some or all of these particular slowdowns (that not everyone is experiencing), the tests will find out!
There's the problem where Max is getting slower with rising number of objects and materials in a scene - scenes with only 30 objects and 15 materials without many maps will not see much of a slowdown. Also, some users are relatively new to Max or unexperienced in general - they might not be able to properly judge what they're seeing.
In this thread, you're getting lots of responses from experienced users (who probably worked with different plugins and renderers in the past) who are seeing this and have reported this for a broad range of Max and Corona versions, so maybe there's some weighting needed whenever users post their experience.
Then there might be hardware issues, or plugins or even user error etc. I understand that looking at a broader picture might make sense.
Still, no reason to doubt the effectivity of using Qt if Autodesk themselves went with it specifically because the UI got slower and it was the only remedy. And moving on from an outdated and limiting framework, of course.