I will likely spend time implementing my solver in several different styles because this is a project I'm tackling largely to make some points about how I think WASM should be used. I'm far from final benchmarks on this but my suspicion is that the gap will be large.

Yes javascript is very well optimized, but as someone who's spent a lot of time writing javascript where speed matters, it's not easy, and it's not predictable. You're at the mercy of arcane optimizations in V8 which might not work for your specific situation because you did something weird, and if you're taking a lot of care not to do anything weird, and manually managing your memory with typed arrays, well, then you might as well write C and compile to WASM.

When it comes to GC languages they can often appear very fast for use cases that don't use a lot of memory.

If you use an algorithm that near exhausts memory, that's where you'll start seeing that "order of magnitude" difference between JS and something like C++. The same goes for Java and C#.

At low memory utilization, the GC can just put off collection, which saves execution time, so the runtime appears fast. But if you're close to the limit, then the GC has no choice but to pause often before continuing. Not very many algorithms will encounter this, but applications might, depending on what they do.