diff --git a/sympl/_core/dataarray.py b/sympl/_core/dataarray.py
index b6b3f84..933786b 100644
--- a/sympl/_core/dataarray.py
+++ b/sympl/_core/dataarray.py
@@ -4,6 +4,7 @@
 
 
 class DataArray(xr.DataArray):
+    __slots__ = []
 
     def __add__(self, other):
         """If this DataArray is on the left side of the addition, keep its
diff --git a/sympl/_core/get_np_arrays.py b/sympl/_core/get_np_arrays.py
index 9a84b57..1cb98d4 100644
--- a/sympl/_core/get_np_arrays.py
+++ b/sympl/_core/get_np_arrays.py
@@ -43,13 +43,16 @@ def get_numpy_array(data_array, out_dims, dim_lengths):
     dict of dim_lengths that will give the length of any missing dims in the
     data_array.
     """
-    if len(data_array.values.shape) == 0 and len(out_dims) == 0:
-        return data_array.values  # special case, 0-dimensional scalar array
+    if len(data_array.data.shape) == 0 and len(out_dims) == 0:
+        return data_array.data  # special case, 0-dimensional scalar array
     else:
         missing_dims = [dim for dim in out_dims if dim not in data_array.dims]
         for dim in missing_dims:
             data_array = data_array.expand_dims(dim)
-        numpy_array = data_array.transpose(*out_dims).values
+        if not all(dim1 == dim2 for dim1, dim2 in zip(data_array.dims, out_dims)):
+            numpy_array = data_array.transpose(*out_dims).data
+        else:
+            numpy_array = data_array.data
         if len(missing_dims) == 0:
             out_array = numpy_array
         else:  # expand out missing dims which are currently length 1.
diff --git a/sympl/_core/restore_dataarray.py b/sympl/_core/restore_dataarray.py
index 791fba1..cf87e90 100644
--- a/sympl/_core/restore_dataarray.py
+++ b/sympl/_core/restore_dataarray.py
@@ -8,9 +8,10 @@
 
 
 def ensure_values_are_arrays(array_dict):
-    for name, value in array_dict.items():
-        if not isinstance(value, np.ndarray):
-            array_dict[name] = np.asarray(value)
+    pass
+    # for name, value in array_dict.items():
+        # if not isinstance(value, np.ndarray):
+            # array_dict[name] = np.asarray(value)
 
 
 def get_alias_or_name(name, output_properties, input_properties):
diff --git a/sympl/_core/units.py b/sympl/_core/units.py
index d2977e8..2278ce0 100644
--- a/sympl/_core/units.py
+++ b/sympl/_core/units.py
@@ -1,22 +1,24 @@
 # -*- coding: utf-8 -*-
+import functools
 import pint
 
 
 class UnitRegistry(pint.UnitRegistry):
-
+    @functools.lru_cache
     def __call__(self, input_string, **kwargs):
         return super(UnitRegistry, self).__call__(
-            input_string.replace(
-                u'%', 'percent').replace(
-                u'°', 'degree'
-            ),
-            **kwargs)
+            input_string.replace(u"%", "percent").replace(u"°", "degree"), **kwargs
+        )
 
 
 unit_registry = UnitRegistry()
-unit_registry.define('degrees_north = degree_north = degree_N = degrees_N = degreeN = degreesN')
-unit_registry.define('degrees_east = degree_east = degree_E = degrees_E = degreeE = degreesE')
-unit_registry.define('percent = 0.01*count = %')
+unit_registry.define(
+    "degrees_north = degree_north = degree_N = degrees_N = degreeN = degreesN"
+)
+unit_registry.define(
+    "degrees_east = degree_east = degree_E = degrees_E = degreeE = degreesE"
+)
+unit_registry.define("percent = 0.01*count = %")
 
 
 def units_are_compatible(unit1, unit2):
@@ -63,9 +65,7 @@ def clean_units(unit_string):
 
 def is_valid_unit(unit_string):
     """Returns True if the unit string is recognized, and False otherwise."""
-    unit_string = unit_string.replace(
-        '%', 'percent').replace(
-        '°', 'degree')
+    unit_string = unit_string.replace("%", "percent").replace("°", "degree")
     try:
         unit_registry(unit_string)
     except pint.UndefinedUnitError:
@@ -75,16 +75,17 @@ def is_valid_unit(unit_string):
 
 
 def data_array_to_units(value, units):
-    if not hasattr(value, 'attrs') or 'units' not in value.attrs:
-        raise TypeError(
-            'Cannot retrieve units from type {}'.format(type(value)))
-    elif unit_registry(value.attrs['units']) != unit_registry(units):
-        attrs = value.attrs.copy()
-        value = unit_registry.Quantity(value, value.attrs['units']).to(units).magnitude
-        attrs['units'] = units
-        value.attrs = attrs
+    if not hasattr(value, "attrs") or "units" not in value.attrs:
+        raise TypeError("Cannot retrieve units from type {}".format(type(value)))
+    elif unit_registry(value.attrs["units"]) != unit_registry(units):
+        out = value.copy()
+        out.data[...] = (
+            unit_registry.convert(1, value.attrs["units"], units) * value.data
+        )
+        out.attrs["units"] = units
+        value = out
     return value
 
 
 def from_unit_to_another(value, original_units, new_units):
-    return (unit_registry(original_units)*value).to(new_units).magnitude
+    return (unit_registry(original_units) * value).to(new_units).magnitude