diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index 8c3a6455d7..18e41c3c3f 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -92,6 +92,9 @@ This document explains the changes made to Iris for this release
 #. `@wjbenfold`_ added caching to the calculation of the points array in a
    :class:`~iris.coords.DimCoord` created using
    :meth:`~iris.coords.DimCoord.from_regular`. (:pull:`4698`)
+#. `@wjbenfold`_ introduced caching in :func:`_lazy_data._optimum_chunksize` and
+   :func:`iris.fileformats.pp_load_rules._epoch_date_hours` to reduce time spent
+   repeating calculations. (:pull:`4716`)
 
 
 🔥 Deprecations
diff --git a/lib/iris/_lazy_data.py b/lib/iris/_lazy_data.py
index 27f09b2a35..ac7ae34511 100644
--- a/lib/iris/_lazy_data.py
+++ b/lib/iris/_lazy_data.py
@@ -10,7 +10,7 @@
 
 """
 
-from functools import wraps
+from functools import lru_cache, wraps
 
 import dask
 import dask.array as da
@@ -47,7 +47,14 @@ def is_lazy_data(data):
     return result
 
 
-def _optimum_chunksize(chunks, shape, limit=None, dtype=np.dtype("f4")):
+@lru_cache
+def _optimum_chunksize_internals(
+    chunks,
+    shape,
+    limit=None,
+    dtype=np.dtype("f4"),
+    dask_array_chunksize=dask.config.get("array.chunk-size"),
+):
     """
     Reduce or increase an initial chunk shape to get close to a chosen ideal
     size, while prioritising the splitting of the earlier (outer) dimensions
@@ -86,7 +93,7 @@ def _optimum_chunksize(chunks, shape, limit=None, dtype=np.dtype("f4")):
     # Set the chunksize limit.
     if limit is None:
         # Fetch the default 'optimal' chunksize from the dask config.
-        limit = dask.config.get("array.chunk-size")
+        limit = dask_array_chunksize
         # Convert to bytes
         limit = dask.utils.parse_bytes(limit)
 
@@ -146,6 +153,25 @@ def _optimum_chunksize(chunks, shape, limit=None, dtype=np.dtype("f4")):
     return tuple(result)
 
 
+@wraps(_optimum_chunksize_internals)
+def _optimum_chunksize(
+    chunks,
+    shape,
+    limit=None,
+    dtype=np.dtype("f4"),
+):
+    # By providing dask_array_chunksize as an argument, we make it so that the
+    # output of _optimum_chunksize_internals depends only on its arguments (and
+    # thus we can use lru_cache)
+    return _optimum_chunksize_internals(
+        tuple(chunks),
+        tuple(shape),
+        limit=limit,
+        dtype=dtype,
+        dask_array_chunksize=dask.config.get("array.chunk-size"),
+    )
+
+
 def as_lazy_data(data, chunks=None, asarray=False):
     """
     Convert the input array `data` to a dask array.
diff --git a/lib/iris/fileformats/pp_load_rules.py b/lib/iris/fileformats/pp_load_rules.py
index 82f40dbf14..c23772f235 100644
--- a/lib/iris/fileformats/pp_load_rules.py
+++ b/lib/iris/fileformats/pp_load_rules.py
@@ -9,6 +9,7 @@
 # SciTools/iris-code-generators:tools/gen_rules.py
 
 import calendar
+from functools import wraps
 
 import cf_units
 import numpy as np
@@ -514,7 +515,7 @@ def _new_coord_and_dims(
 _HOURS_UNIT = cf_units.Unit("hours")
 
 
-def _epoch_date_hours(epoch_hours_unit, datetime):
+def _epoch_date_hours_internals(epoch_hours_unit, datetime):
     """
     Return an 'hours since epoch' number for a date.
 
@@ -589,6 +590,30 @@ def _epoch_date_hours(epoch_hours_unit, datetime):
     return epoch_hours
 
 
+_epoch_date_hours_cache = {}
+_epoch_date_hours_cache_max_size = 128  # lru_cache default
+
+
+@wraps(_epoch_date_hours_internals)
+def _epoch_date_hours(epoch_hours_unit, datetime):
+    # Not using functools.lru_cache because it does an equality check that fails
+    # on datetime objects from different calendars.
+
+    key = (epoch_hours_unit, hash(datetime))
+
+    if key not in _epoch_date_hours_cache:
+        _epoch_date_hours_cache[key] = _epoch_date_hours_internals(
+            epoch_hours_unit, datetime
+        )
+
+        # Limit cache size
+        while len(_epoch_date_hours_cache) > _epoch_date_hours_cache_max_size:
+            oldest_item = next(iter(_epoch_date_hours_cache))
+            _epoch_date_hours_cache.pop(oldest_item, None)
+
+    return _epoch_date_hours_cache[key]
+
+
 def _convert_time_coords(
     lbcode,
     lbtim,