Admittedly, building Qt WebAssembly apps takes what seems like forever. Technically, it is the linking part that takes a huge amount of time, as that is where the magic happens.
You may have missed my first blog regarding faster build times for Qt WebAssembly apps:
Here is how to get set up to use this now integrated feature.
./emsdk install latest
./emsdk activate --embedded latest
You can then configure and rebuild Qt with -device-option WASM_OBJECT_FILES=1
configure -xplatform wasm-emscripten -developer-build -nomake tests -nomake examples -opensource -confirm-license -verbose -compile-examples -no-warnings-are-errors -release -device-option WASM_OBJECT_FILES=1
Then you can build your Qt app as normal using the qmake from that build, and watch the build time speed up!
*NOTE* Using WASM_OBJECT_FILES with a multi-threaded build is still experimental, as both threads and using wasm object files in emscriptem/wasm and Qt are experimental. I have not seen any real issues regarding the use of both of these at the same time.
You can read more about Qt for WebAssembly, mobile and embedded development in the book Hands-On Mobile and Embedded Development with Qt 5