[acdha]

I'd also expect unrelated load to be a factor, especially if there's a mechanism which priorities paid customer traffic over the free tier. Given the complexities of loading and running JavaScript, I'd be especially interested in what memory contention looks like — it seems like it could be as simple as Rust needing far less memory than loading all of SpiderMonkey and thus being less likely to hit both low-level CPU cache and high-level runtime cache evictions.

[azakai]

Ah, good point, memory consumption might indeed be a factor here. If that's the case I'd expect higher variance in the measurements perhaps - looking for that in the raw data could be interesting.