I then saw that rcppsimdjson doesnt currently support writing, but if they ever extend to writing, that might be used for writing the R data to a data file used for sampling.
cmdstanr will use JSON for the data file. It was already using it previously, but we have temporarily switched back to Rdump because of the multi dimensional zero sized arrays issue with JSON in cmdstan. that has since been squashed. It will again use JSON once https://github.com/stan-dev/cmdstanr/pull/110 is merged. We are currently using jsonlite but if this package would look promising we might think abou switching.
Given that simdjson is supposed to be the fastest C++ JSON library its definitely promising.