diff --git a/CHANGELOG b/CHANGELOG
index 87642f85..c49000a3 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,4 +1,5 @@
 0.54.3
+ - enh: more caching of event size and shape for HDF5 format
  - enh: faster computation of contour length for DCOR format
  - fix: implement `__contains__` for DCOR logs and tables
  - enh: use dcserv version 2 in DCOR format (fast S3 access)
diff --git a/dclab/rtdc_dataset/fmt_hdf5/events.py b/dclab/rtdc_dataset/fmt_hdf5/events.py
index bf2cbf86..e98df69e 100644
--- a/dclab/rtdc_dataset/fmt_hdf5/events.py
+++ b/dclab/rtdc_dataset/fmt_hdf5/events.py
@@ -38,7 +38,6 @@ def __iter__(self):
 
     def __len__(self):
         if self._length is None:
-            assert False
             # computing the length of an H5Group is slow
             self._length = len(self.h5group)
         return self._length
@@ -207,6 +206,8 @@ def shape(self):
 class H5TraceEvent:
     def __init__(self, h5group):
         self.h5group = h5group
+        self._num_traces = None
+        self._shape = None
 
     def __getitem__(self, idx):
         return self.h5group[idx]
@@ -215,7 +216,9 @@ def __contains__(self, item):
         return item in self.h5group
 
     def __len__(self):
-        return len(self.h5group)
+        if self._num_traces is None:
+            self._num_traces = len(self.h5group)
+        return self._num_traces
 
     def __iter__(self):
         for key in sorted(self.h5group.keys()):
@@ -226,5 +229,8 @@ def keys(self):
 
     @property
     def shape(self):
-        atrace = list(self.h5group.keys())[0]
-        return tuple([len(self.h5group)] + list(self.h5group[atrace].shape))
+        if self._shape is None:
+            atrace = list(self.h5group.keys())[0]
+            self._shape = tuple([len(self)] + list(self.h5group[atrace].shape))
+        return self._shape
+