I add a widget via the Customizer. It's a simple widget, with basic loop, which displays 10 latest posts where each post have Featured Image. Because of the Featured Image, this widget run hundreds of queries, without Featured Image the number of queries returns back to normal.
It happens only when adding widgets and viewing the site via the Customizer, there is no such issue on the front end.
Also tested with the default WP Recent Posts widget, the result was the same.
Have anyone experienced something similar?