toJSON is very slow compared to using rjson's

[nocodehummel]

The default Shiny toJSON (jsonlite) is very slow compared to rjson's toJSON, which is approximately 25x faster in microbench tests (highcharter object with AAPL YTD quotes).

In my test I found that rjson::toJSON converts NA's to strings, which cannot be plotted by Highcharts (Javascript error 14 in browser console). After removing NA's my charts plot without problems; including OHLC stock, line, stacked bars and scatter.

Suggest to add option to use rjson::toJSON to improve performance.

[nocodehummel]

Closed after merge into master.

[hongyuanjia]

@nordicgit70 #457 guided me here. Thanks for this. I wonder if you could try https://github.com/eddelbuettel/rcppsimdjson, which is I believe the fastest json serialization library for R.