View validation accounts for a fair cost of module installation, most of
that is spent checking for view validity, and a significant fraction
of *that* is spent reading the validated view from the DB.
Turns out _check_xml requested *all* view fields even though it needed
none of them save for the arch. Specifying fields to read_combined
rather than fetching all fields seems to result in a ~30% speedup of
_check_xml (under line_profiler). Most of the rest is spent fetching
sub-views (in get_inheriting_view_arch), I don't know that it can be
improved without hand-crafting a fairly complex SQL request.