From the official bug tracker [1], I found that such behavior can be caused by full-text search indexing. And what? As soon as I went to settings and disabled full-text search, CPU usage dropped to zero. Moreover, while dictionaries are being indexed for full-text search, there is a label at the bottom of the main window telling this. So, I think that this report should be closed.
[1] https://github.com/goldendict/goldendict/issues/640
