DOC Optimizing WASM browser compilation time

The documentation on [optimizing compilation](https://emscripten.org/docs/optimizing/Optimizing-Code.html) and optimizing [load time](https://emscripten.org/docs/compiling/Deploying-Pages.html#optimizing-page-startup-times) are quite nice.

I wonder if anyone could share optimization advice specifically for the WASM compilation time in the browser. Are there options that are known to affect it, well besides the size of the produced .wasm objects? We found out that it's the bottleneck in Pyodide for large scientific Python packages (https://github.com/pyodide/pyodide/issues/347#issuecomment-933834657)

Also the documentation on the manual persistance of compiled modules to IndexedDB might need to be updated,
https://github.com/emscripten-core/emscripten/blame/2595d3f703b6d6eaf312a2b027ef66c6ab3c1031/site/source/docs/compiling/Deploying-Pages.rst#L44
since that was deprecated https://github.com/WebAssembly/spec/issues/821