Hi Jeff Brewer,
Re: "I'm trying out live captions for the first time. Seems to stop working after a few seconds, misses a few sentences, then starts again for a bit. Very sporadic, and not particularly accurate.
Seems others also had problems with iOS 18: See: ios 18 live captions have few problems - Apple Community
Note: Live captions first appeared as "beta" software (kind of on trial)
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Re: Wondering if this is just not very good or maybe there are ways to enhance its performance?"
Wondering which iOS version is installed on your iPhone: Eg: About iOS 18 Updates - Apple Support
says: iOS 18.3.1 is the newest version and
"This update provides important bug fixes and security updates and is recommended for all users".
You can try checking for updates in Settings > General > Software Updates.
NB: Please remember: Before updating it is wise to back-up your iPhone
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If you already had iOS 18.3.1 installed, before you had this issue:
Wondering whether you have tried re-starting your iPhone
(ie: deletes temporary files, re-establishes connections, may update recent setting changes, and often restores order if something is not working as expected.)
Apple Support's online article may have tips: Get live captions of spoken audio on iPhone - Apple Support
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If updating or restarting hasn't helped:
Seems best to send feedback to Apple ... although it won't help immediately, and they will likely not reply, your feedback is valuable. Your feedback will be carefully read and logged. If others have a similar issue, Apple can see which iPhones and iOS versions are affected. We may see the issue addressed in an update.
Web-form for Feedback - iPhone - Apple
All the best :-)