(When I was investigating this it was very early in Node’s `--import` story, but there were several edge cases with the more common ESM-to-CJS approaches that I wanted to address. Most were probably exceedingly niche concerns, but I’d expect top-level await to affect a meaningful subset of users.)
Relatedly Node.js recently (2025) introduced a synchronous version of its resolver hook registration API (`module.registerHooks()`) specifically to improve performance over the old async `module.register()` API. It was a big unblocker for Nub. For the interested, the async API added 19ms fixed registration overhead + about 130us additional overhead per import.
Which flag Nub uses here doesn't impact userland at all, TLA is supported wherever it's supported by Node.js itself.