WebEngine performance issues with 5.15.8

After switching from Qt 5.15.2 to 5.15.8, WebEngine performance is significantly slower than before (probably related to JavaScript). I've attached an HTML file that demonstrates this behavior.

I used the simplebrowser example project and loaded this HTML file from a HTTP server on the same machine, and added some debug code to display elapsed time together with loadProgress.

Loading the attached file for the first time takes always less than 1 second with 5.15.2 and always more than 2 seconds with 5.15.8 - this only happens the first time the file is loaded, reloading takes about the same time with both versions