Anyway, thanks to some help by a lovely local duck, Torii has an experimental WASM-based simulation backend!
For smaller simulations pysim is still faster, but for *huge* sim tests it's a good improvement over pysim!
https://github.com/shrine-maiden-heavy-industries/torii-sim-wasm
I also eventually plan to try to properly get a Verilator-backed simulation engine built up too,
Which *should* be faster than the WASM engine for the cases where it matters, but is less portable due to the fact your system needs both Verilator and a C++ compiler.
Whereas with the WASM backend as it stands it's pretty much portable out of the box for all the platforms that wasmtime supports.
There is still a bunch of perf tuning to do so we can get some more nyoomies out of it, but it's already a huge improvement for some of the super big test benches.