> In short, OSSL_PARAM works by passing arrays of key-value pairs to functions, instead of normal argument passing. 

I think one of the main motivators was supporting the new module framework that replaced engines. The FIPS module specifically is OpenSSL's gravy train, and at the time the FIPS certification and compliance mandate effectively required the ability to maintain ABI compatibility of a compiled FIPS module across multiple major OpenSSL releases, so end users could easily upgrade OpenSSL for bug fixes and otherwise stay current. But OpenSSL also didn't want that ability to inhibit evolution of its internal and external APIs and ABIs.

Though, while the binary certification issue nominally remains, there's much more wiggle room today when it comes to compliance and auditing. You can typically maintain compliance when using modules built from updated sources of a previously certified module, and which are in the pipeline for re-certification. So the ABI dilemma is arguably less onerous today than it was when the OSSL_PARAM architecture took shape. Today, like with Go, you can lean on process, i.e. constant cycling of the implementation through the certification pipeline, more than technical solutions. The real unforced error was committing to OSSL_PARAMs for the public application APIs, letting the backend design choices (flexibility, etc) bleed through to the frontend. The temptation is understandable, but the ergonomics are horrible. I think performance problems are less a consequence of OSSL_PARAMS, per se, but about the architecture of state management between the library and module contexts.