CarpetIOHDF5 has support for writing an "index" file alongside the normal HDF5 output files. This is an HDF5 file in which the datasets are present but contain no data. This index file is much faster to read than the full HDF5 file (due to locality of data in the index file and block reads on slow filesystems) and can significantly speed up analysis of the data.
Index files are currently written for the "standard" (typically 3D) data in Output.cc, but are not written for the "sliced" data in OutputSlice.cc. Since visualisation of 2D data is very common, OutputSlice.cc should be modified to output index files as well.