add some notes about JIT compilation issues

[cdetrio]

[gcolvin]

Wasm is of course designed such that reasonably optimized machine code can be generated in n log n time. And it can be generated with a simpler compiler than usual. So I don't think it's too much of a stretch for high-performance clients to provide at least a minimal compiler. Wasm's design also makes high-speed interpreters possible in system-level languages like C and Rust, so compilers aren't the only option.