QML performance regression when accessibility is active

We have recently migrated our statically linked audio plugin application from 5.12.8 to 5.15.8. We are experience a severe performance regression when accessibility is active.

Profiling the application indicates that the regression is occurring in QAccessible::queryAccessibleInterface whilst parsing static plugin metadata.

This looks to have been introduced by this commit introduced as a fix for QTBUG-75106.

Prior to the commit, classes with no QAccessiblePlugin were inserted into the qAccessiblePlugins cache regardless and their metadata would not be parsed the next time this class was queried.

This is a blocker for us  - we are seeing an unresponsive UI at times > 20 seconds. Many of our users use productivity apps that utilize the OSX Accessibility API (e.g. Cinch, Default Folder X).