The SWC-based transpilation looks like another big improvement. For anyone else who had a similar configuration to us, with some Babel packages installed to support Jest for running tests regardless of what Parcel was doing with the main build process, this also looks interesting, though we haven’t tried it yet:
https://swc.rs/docs/usage-swc-jest/
Edit: We have now tried it, but sadly several tests broke due to parsing problems. The cause remains a mystery so far, because the offending lines are in our application code and not our test files, and that same code does appear to build OK via Parcel. So it appears that SWC itself, at least as Parcel is using it, can parse everything involved, but for some reason to be determined, installing @swc/* packages directly as devDependencies and configuring the Jest transform for the relevant source files to use @swc/jest is not a drop-in replacement for the Jest+Babel combination we were using before. If anyone else has had more luck getting this working, and similarly getting ESLint to work well without needing Babel packages after moving other tools to use SWC, details would be appreciated! Otherwise, it looks like we’ll still need Babel for now to run the testing and linting tools, even if Parcel is using SWC for its builds.