Simple QWebSocket app hangs in WebAssembly

I have a simple gui websocket app modelled on qtwebsockets/examples/websockets/echoclient

When built for Windows with Qt 5.7.0 the app connects to the server, gets a response and displays it. When built for WebAssembly with Qt 6.3.1 the app hangs as soon as the socket is created with the browser reporting "This page is slowing down Firefox. To speed up your browser, stop this page."

Emscripten is v3.0.0 and Qt 6.3.1 for WebAssembly was configured with:

./configure -qt-host-path /opt/Qt/6.3.1/gcc_64 -platform wasm-emscripten -prefix /opt/Qt/6.3.1-wasm/qtbase -device-option QT_EMSCRIPTEN_ASYNCIFY=1

Source code for the project is attached.