Relative to e W w , computing e C w is computational expensive, as it requires two LSTMs traversals of length m. However, e C w only depends on the character sequence of that word, which means that unless the parameters are updated, it is possible to cache the value of e C w for each different w's that will be used repeatedly.
the model can keep a list of the most frequently occurring word types in memory and run the compositional model only for rare words.