reading data frame stored in HDF5

I have a large data-frame (126041 Obs. of 604 variables). I'm new to HDF5 formats. I save the HDF5 file as follows:

writeH5DataFrame(myData,"C:/myDir/myHDF5.h5",overwrite=T)

1. how can I read the data frame back? there doesn't appear to be any readH5DataFrame or loadH5DataFrame function?

2. also, the writeH5DataFrame takes an incredibly long time, probably because of the large number of columns (604 in this case). The documentation mentions that "the data for each column is stored in a separate H5Dataset." - not sure if this the reason for the long time taken. Is there any way to speed up writing a DataFrame in HDF5 format?