Load performance improvement (ignoring UGRID) (SciTools#6088)

* proof of concept load performance improvement

* remove print line

* adjust for merge

* adjust to tolerant load behaviour, make methods private

* fix tests

* simplify changes

* Update lib/iris/fileformats/cf.py

Co-authored-by: Bill Little <bill.little@metoffice.gov.uk>

* address review comment

* add whatsnew

---------

Co-authored-by: Bill Little <bill.little@metoffice.gov.uk>

docs/src/whatsnew/latest.rst

@@ -98,6 +98,10 @@ This document explains the changes made to Iris for this release
    subclasses of a common generic :class:`~iris.mesh.components.Mesh` class.
    (:issue:`6057`, :pull:`6061`, :pull:`6077`)
 
+#. `@pp-mo`_ and `@stephenworsley`_ Turned on UGRID loading by default, effectively removing
+   the need for and deprecating the :func:`~iris.ugrid.experimental.PARSE_UGRID_ON_LOAD`
+   context manager. (:pull:`6054`, :pull:`6088`)
+
 
 🚀 Performance Enhancements
 ===========================

lib/iris/fileformats/cf.py

@@ -1331,6 +1331,11 @@ def __init__(self, file_source, warn=False, monotonic=False):
 
         self._check_monotonic = monotonic
 
+        self._with_ugrid = True
+        if not self._has_meshes():
+            self._trim_ugrid_variable_types()
+            self._with_ugrid = False
+
         self._translate()
         self._build_cf_groups()
         self._reset()
@@ -1348,6 +1353,25 @@ def __exit__(self, exc_type, exc_value, traceback):
         # When used as a context-manager, **always** close the file on exit.
         self._close()
 
+    def _has_meshes(self):
+        result = False
+        for variable in self._dataset.variables.values():
+            if hasattr(variable, "mesh") or hasattr(variable, "node_coordinates"):
+                result = True
+                break
+        return result
+
+    def _trim_ugrid_variable_types(self):
+        self._variable_types = (
+            CFAncillaryDataVariable,
+            CFAuxiliaryCoordinateVariable,
+            CFBoundaryVariable,
+            CFClimatologyVariable,
+            CFGridMappingVariable,
+            CFLabelVariable,
+            CFMeasureVariable,
+        )
+
     @property
     def filename(self):
         """The file that the CFReader is reading."""

lib/iris/fileformats/netcdf/ugrid_load.py

@@ -56,11 +56,13 @@ def _meshes_from_cf(cf_reader):
     # Mesh instances are shared between file phenomena.
     # TODO: more sophisticated Mesh sharing between files.
     # TODO: access external Mesh cache?
-    mesh_vars = cf_reader.cf_group.meshes
-    meshes = {
-        name: _build_mesh(cf_reader, var, cf_reader.filename)
-        for name, var in mesh_vars.items()
-    }
+    meshes = {}
+    if cf_reader._with_ugrid:
+        mesh_vars = cf_reader.cf_group.meshes
+        meshes = {
+            name: _build_mesh(cf_reader, var, cf_reader.filename)
+            for name, var in mesh_vars.items()
+        }
     return meshes