It’s perhaps telling that they had an initial prototype which was 2x slower, then talk about specific optimization strategies but never share the updated overall speed comparison.
https://workspace.google.com/blog/sheets/new-innovations-in-...
"Building on improvements like smooth scrolling and expanded cell limits in Sheets, today we’re announcing that we’ve doubled the speed of calculation in Sheets on Google Chrome and Microsoft Edge browsers,"...
I assume this is 2x the speed of their current JavaScript code.
The good news is that most of those optimizations weren't specific to Java, like work in V8 (that helps any Chromium-based-browser running WasmGC) and work in Binaryen (that helps any Binaryen-using toolchain, like Kotlin and Dart).