TL;DR; it's possible, but lots of work, and not that huge of a gain in the context of a Ruby JSON parser.
`ruby/json` doesn't use explicit SIMD instructions, some routines are written in a way that somewhat expects compilers to be able to auto-vectorize, but it's never a given.
In theory using SIMD would be possible as proven by SIMDjson, but it's very (edit) UNlikely we'll do it because of multiple reasons.
First for portability, we have to stick with raw C99, no C++ allowed, so that prevent using SIMDjson outright.
In theory, we could implement the same sort of logic with support for various processors that have various level of SIMD support and have runtime dispatch for it would be terribly tedious. So it's not a reasonable amount of complexity for the amount of time I and other people are willing to spend on the library.
Then there's the fact that it wouldn't do as big as a difference as you'd think. I do happen to have made some bindings for simdjson in https://github.com/Shopify/heap-profiler, because I had an use case for parsing gigabytes of JSON, and it helps quite a bit there.
But I'll hopefully touch on that in a future blog post, the actual JSON parsing part is entirely dwarfed by the work needed to build the resulting Ruby objects tree.