7_Networks.ipynb unusably slow?

Maybe it's just my system somehow, but the interactive plot at the end of 7_Networks.ipynb is unusably slow, and I assume it hasn't always been that way, or else we would have drastically simplified the example. Right now the Bokeh plot updates seemingly much more quickly than the Datashader version, which takes *minutes* to redraw on pan or zoom and leaves it looking like this in the meantime:


Start with 7_Networks.ipynb and reproduce the interactive plot's pan and zoom behavior, comparing the Bokeh and Datashader versions. Investigate why the Datashader plot takes minutes to redraw; done means the reported slowdown is resolved or its cause and limits are clearly established.

Written by the indexing model from the issue text.