Tokio is a general purpose async runtime. Much the same could probably be said for async-std (except IIRC they do have a barebones reactor for you to build your own on). In general, a general-purpose async runtime will do worse for highly specific tasks than a purpose-built one (especially e.g. NUMA).

I think avoiding async entirely might be a mistake, and I'm not entirely convinced anything better than a general-purpose async runtime might exist for a JS runtime (it itself is general purpose after all).

Avoiding std::fs is fucking bizarre to me: it's completely sync and is a really lightweight abstraction over syscalls.