diff --git a/newsfragments/881.bugfix b/newsfragments/881.bugfix
new file mode 100644
index 000000000..41b81e916
--- /dev/null
+++ b/newsfragments/881.bugfix
@@ -0,0 +1 @@
+Cache detector module slices in FormatNXmx to avoid redundant HDF5 reads
diff --git a/src/dxtbx/format/FormatNXmx.py b/src/dxtbx/format/FormatNXmx.py
index 6963d5f5f..af71fac61 100644
--- a/src/dxtbx/format/FormatNXmx.py
+++ b/src/dxtbx/format/FormatNXmx.py
@@ -116,6 +116,9 @@ def _start(self):
             dxtbx.nexus.get_static_mask(nxdetector)
         )
         self._bit_depth_readout = nxdetector.bit_depth_readout
+        self._nxdata_cached = nxdata
+        self._nxdetector_cached = nxdetector
+        self._module_slices_cached = dxtbx.nexus.get_detector_module_slices(nxdetector)
 
         if self._scan_model:
             self._num_images = len(self._scan_model)
@@ -158,11 +161,12 @@ def get_static_mask(self, index=None, goniometer=None):
         return self._static_mask
 
     def get_raw_data(self, index):
-        nxmx_obj = self._get_nxmx(self._cached_file_handle)
-        nxdata = nxmx_obj.entries[0].data[0]
-        nxdetector = nxmx_obj.entries[0].instruments[0].detectors[0]
         raw_data = dxtbx.nexus.get_raw_data(
-            nxdata, nxdetector, index, bit_depth=self._bit_depth_readout
+            self._nxdata_cached,
+            self._nxdetector_cached,
+            index,
+            bit_depth=self._bit_depth_readout,
+            module_slices=self._module_slices_cached,
         )
         if self._bit_depth_readout:
             # if 32 bit then it is a signed int, I think if 8, 16 then it is
diff --git a/src/dxtbx/format/FormatNXmxEigerFilewriter.py b/src/dxtbx/format/FormatNXmxEigerFilewriter.py
index 71d5577dd..492650ae3 100644
--- a/src/dxtbx/format/FormatNXmxEigerFilewriter.py
+++ b/src/dxtbx/format/FormatNXmxEigerFilewriter.py
@@ -92,16 +92,18 @@ def _get_nxmx(self, fh: h5py.File):
         return nxmx_obj
 
     def get_raw_data(self, index):
-        nxmx_obj = self._get_nxmx(self._cached_file_handle)
-        nxdata = nxmx_obj.entries[0].data[0]
-        nxdetector = nxmx_obj.entries[0].instruments[0].detectors[0]
-
         # Prefer bit_depth_image over bit_depth_readout since the former
         # actually corresponds to the bit depth of the images as stored on
         # disk. See also:
         #   https://www.dectris.com/support/downloads/header-docs/nexus/
         bit_depth = self._bit_depth_image or self._bit_depth_readout
-        raw_data = get_raw_data(nxdata, nxdetector, index, bit_depth)
+        raw_data = get_raw_data(
+            self._nxdata_cached,
+            self._nxdetector_cached,
+            index,
+            bit_depth,
+            module_slices=self._module_slices_cached,
+        )
 
         if bit_depth:
             # if 32 bit then it is a signed int, I think if 8, 16 then it is
@@ -122,6 +124,7 @@ def get_raw_data(
     nxdetector: nxmx.NXdetector,
     index: int,
     bit_depth: int | None = None,
+    module_slices: tuple[tuple[slice, ...], ...] | None = None,
 ) -> tuple[flex.float | flex.double | flex.int, ...]:
     """Return the raw data for an NXdetector.
 
@@ -142,9 +145,11 @@ def get_raw_data(
         raise IndexError(f"Out of range index for raw data {index}")
     all_data = []
     sliced_outer = data[index]
-    for module_slices in get_detector_module_slices(nxdetector):
+    if module_slices is None:
+        module_slices = get_detector_module_slices(nxdetector)
+    for slices in module_slices:
         data_as_flex = _dataset_as_flex(
-            sliced_outer, tuple(module_slices), bit_depth=bit_depth
+            sliced_outer, tuple(slices), bit_depth=bit_depth
         )
         all_data.append(data_as_flex)
     return tuple(all_data)
diff --git a/src/dxtbx/nexus/__init__.py b/src/dxtbx/nexus/__init__.py
index 8dc71d88c..b1aa24a5d 100644
--- a/src/dxtbx/nexus/__init__.py
+++ b/src/dxtbx/nexus/__init__.py
@@ -566,6 +566,7 @@ def get_raw_data(
     nxdetector: nxmx.NXdetector,
     index: int,
     bit_depth: int | None = None,
+    module_slices: tuple[tuple[slice, ...], ...] | None = None,
 ) -> tuple[flex.float | flex.double | flex.int, ...]:
     """Return the raw data for an NXdetector.
 
@@ -583,9 +584,11 @@ def get_raw_data(
         data = list(nxdata.values())[0]
     all_data = []
     sliced_outer = data[index]
-    for module_slices in get_detector_module_slices(nxdetector):
+    if module_slices is None:
+        module_slices = get_detector_module_slices(nxdetector)
+    for slices in module_slices:
         data_as_flex = _dataset_as_flex(
-            sliced_outer, tuple(module_slices), bit_depth=bit_depth
+            sliced_outer, tuple(slices), bit_depth=bit_depth
         )
         data_as_flex.reshape(
             flex.grid(data_as_flex.all()[-2:])