Very slow saving large widget

[dmurdoch]

This issue: dmurdoch/rgl#143 describes a situation in which htmlwidgets::saveWidget is very, very slow. I've traced into it, and it appears the slowness is all in Pandoc: the widget data is in a very long string with one entry, and this is sent to Pandoc, which takes a long time to process it.

Is there a way to save the data so that it doesn't slow down so much? It's currently a big JSON object containing lots of arrays of numbers converted to characters.