Optimizing for code size by default may hurt performance severely

[alexcrichton]

A topic that recently came up in #wg-wasm on Discord was that a wasm-pack project was performance much more slowly than a raw rustc-generated wasm file. It turned out to be related to opt-level = 's' in Cargo.toml, causing a nearly 3x (!) slowdown compared to -C opt-level=3 (the default).

I'm not sure how often this comes up in the wild though. Unfortunately there's not really a silver bullet here :(. I figured it'd be good to catalog though!