We’ve done some more testing.
The performance was much better when we switched from integrated to dedicated graphics, and was likely what was used for the demo video in the pull request.
It was also much faster when we changed the the high dpi scaling settings to system (at the cost of resolution).
It’s still not obvious what the difference is that makes Slicer faster than the example application. It may be a congestion of events from the gestures that are causing the performance issue.