That's Dune convention.

This saves another 15% run-time, at least in debug builds.

This saves about 20% run time.

dune-grid 2.9 introduced the Geometry::jacobianInverse method,
which  allows to transform shape function Jacobians from
reference element coordinates to world coordinates in a much
simpler way.

That's easy for the element stiffness matrix.  For the other matrix
and vector types it has to wait, because not everything that we use
from dune-istl support BCRSMatrix<double> yet.

They are not actually used.

Computing the material phase at a particular point may involve
a call to a Python function.  That is usually quite costly.
Therefore, make sure that this call is really made only once
per quadrature point.

Because such statements bleed into all the files that include
the headers, and this may lead to compilation errors later.