Running the Last.fm importer script, with no other applications accessing ListenBrainz or MusicBrainz at the same time. I had the browser dev tools open and noticed a number of 429 Rate Limit Exceeded responses which weren't causing a slowdown in outgoing requests, and the failed requests don't appear to have been retried. The UI shows no indication of errors that have occurred (at least not currently, while the import is still running.