@maximo Depending on needs of your application you may create a customized build of QtWebKit without some features, which may substantially reduce binary size.
Since you are using an unreleased version of Qt and hitting something that's probably influenced by the internals of Qt. I'd recommend posting your question on the interest mailing list You'll find there Qt's developers/maintainers (this forum is more user oriented)
I discovered, your account is moderator type (I think), my account is a normal user, so I'm not such a function, but @tekojo had an idea to use "tags" instead of this function, see: https://bugreports.qt.io/browse/QTWEBSITE-631