QWebEngine: massively slower in 5.7.x

When I upgraded from Qt 5.6.2 to Qt 5.7.1 (I skipped 5.7.0 because of a bug that prevented it from being usable to me) I got a massive slowdown in QWebEngine. Loading of pages, AND rendering, is now significantly slower than before. We're not talking about 2 or 3 times slower. We're talking about 8 to 10 times slower (or so it feels). 

This is in debug mode.

This has forced me to compile in Release mode, because that's the only way I get acceptable performance. Release mode now feels 'about the same' as Debug mode felt in Qt 5.6.2, which is disappointing, because I had hoped to get a speedup when switching to Release mode. 

The worst part of this is that it's not a bit more difficult to debug my application, because I am in Release mode all the time. It's quite simply too painful to use in Debug.