We pre-compute this distribution off-line for each token so that the added complexity is proportional to the size of the vocabulary.
precomputing the co-occurrences and graphs using corpus2graph (Zhang et al., 2018) takes about 2.5 hours on top of 8 hours for word2vec alone on the entire Wikipedia corpus using 50 logical cores on a server with 4 Intel Xeon E5-4620 processors : the extra cost is not excessive.