Results dialog window opening very slowly

This problem wasn't very complicated but it was rather serious and its solution was somehow embarrassing, so let's have a look at it.

Context

I was developing a program for migrating some application's data from the legacy format to the format used by its newest version. Needless to say, I had to provide a UI for it as well, so I wrote a modern-looking slim GUI using QtWidget. There was a start screen where the migration process could be parameterized and started, the status screen where the progress of the operation was visualized, and the result window where detailed information about each of the migration subtasks would be given.

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