Assembler, but not at all

[parisse]

I don't think one should consider RPL, this was a language designed for calculators with hardware available in 1980. In order to have acceptable performances for the 49 CAS, I had to code in system RPL, code with the stack and avoid local variables as much as possible, use unnamed local variables (NULLNAME), and critical sections required assembly code. This gives code that is hard to create and maintain and is not portable.
Programming in RPL was reserved to an elite, and that's perhaps a reason why some people like(d) RPL...

WebAssembly is much better. Giac compiled to wasm is about 2* to 3* slower than native compiled code (depends what you are doing, for floating point operations it's sometimes as fast as native code), that's far better than any interpreter. And you could use whatever language compiles to wasm.