Likely because they are compiling Java with WasmGC extensions and stuff. If you try C, Rust, Zig etc they tend to run extremely fast in WASM.
Go also has a WASM target which runs pretty slow compared to its native binaries. GC extensions might help but as far as I can remember, Go's memory model does not fit the WasmGC so it might never be implemented.
I'm somewhat curious why they even chose Java over Rust or Zig, considering the spreadsheet data paths should be relatively straight forward even with a lot of clone() activity.
Go also has a WASM target which runs pretty slow compared to its native binaries. GC extensions might help but as far as I can remember, Go's memory model does not fit the WasmGC so it might never be implemented.