I don't see how the link clarifies things. simdjson is fast enough that JSON decoding can approach single-core bandwidth capacity, but it doesn't follow that a JSON-heavy load is bandwitdh constrained. In fact the existence of simdjson points to json decoding being compute bound.

It’s a bit more nuanced than that. Scalar code that does lots of branching and loads one value at a time (e.g. single character) often can’t hit memory at peak bandwidth rate. On the other hand well vectorized code that loads vectors of multiple values at a time can properly saturate the available hardware resources.

The earlier post was referring to "a typical REST server" in a context of the claim that memory bandwidth "is becoming the ultimate spec".

simdjson (impressive as it may be) existing as an interesting project seems likely to imply that most JSON parsing isn't done with such performant methods on "the typical REST server".

EDIT: Seeing that it's used in many projects including Node.js shifts me back to thinking more highly of the claim that memory bandwidth is becoming the ultimate spec!