This prevents the Python GIL from being released before calling the method
(which is the default behaviour). For very cheap to call c++ methods the
cost of releasing the GIL can outweigh the cost of the c++ call, which means
it's more efficient to retain the hold on the GIL.
Ideally we'd do this everywhere, and switch to an explicit ReleaseGIL
annotation on functions which are slow or risky (raise exceptions, or
do something which can cause a GIL deadlock). But those are very tricky
to identify, so instead just explicitly hold the gil on cheap methods
which are likely to be called many times and could have an impact on
script performance.
All your uses of toUtf8().data() actually just need a const char*
So use constData() that is semantically more correct, and documented
to be faster.
From http://doc.qt.io/qt-5/qbytearray.html#data
"For read-only access, constData() is faster because it never
causes a deep copy to occur."
