Not dumb questions at all.

Multicore adds parallelism via Domains (which are essentially heavyweight threads) and concurrency via Effects and fibers. There's a multicore GC that supports both of those.

We plan to upstream things in two parts. First domains-only parallelism and then effects as a follow-up. When the latter lands users will be able to define their own effects and handlers, yes.

Performance is pretty good, you can see our PLDI2021 paper for a proper performance evaluation and loads more details: https://arxiv.org/abs/2104.00250