But the HDF5 library does not really support multi-threading at all. Compiling the library with the "threading" option just locks around every API call, so you'

But the HDF5 library does not really support multi-threading at all. Compiling the library with the "threading" option just locks around every API call, so you're back to a single thread whenever you enter (compiling without it will just crash your program). And the library does quite a lot of work when you call into it; chunk lookup, decompression, and type conversions all happen behind that lock. You can use the "direct chunk access" functions (H5Dread_chunk?) to bypass a lot of that work and do it yourself, so you get back to using multiple threads again, and that can be a big win, but having to do it sucks, and I don't think h5py exposes this functionality at all.