From Rene Ritchie’s iOS 7 review:
What Apple hasn’t added is any local, on-device functionality for Siri. Google’s been doing this for a while, and it helps minimize network connections and backend servers as a point of failure. Basically, for any action that involves only the apps on the phone or tablet, for example, toggling a setting or adding a reminder, all voice parsing is done on the device. Only when a request needs to go online, like to check the web or check with a service, does it hit servers. Siri currently goes to the servers for everything, making it slower and subject to more failures than Google’s voice tech.
Before the introduction of Siri, Voice Control in iOS 3 was capable of processing simple commands like playing specific songs or artists, adding something to a Genius playlist, calling a person in your Contacts, and reading back the time when requested. The 3GS had enough processing power to be able to do all of these things off-line.
Siri debuted on the iPhone 4, which was even more powerful than its predecessor. But, there was no mode for failing gracefully when no network connection was available, not even the simple voice commands that previously worked completely without networking while running on less capable hardware. Siri will churn for a few seconds and then recite a canned (sometimes sassy) response. Siri was billed as “beta” software, and seems to have deserved that appellation.
Whenever possible, graceful failure modes should be implemented. For some reason, no one at Apple thought that it was important for Siri to be able to work offline, even though in the past Voice Command worked well with no network connection at all. Access to information that is available locally on the iPhone itself should be able to work without a connection. Whenever possible, Siri should default to working locally first, before sending server requests.
Google was ahead of Apple in speech processing when Siri was announced; what they lacked was system integration in iOS. I wanted to like Siri, but I found that using it was too unreliable and its failure mode resulted in an awful user experience. That may have gotten better with the new iOS 7 version of Siri, but it remains to be seen. Right now, it looks like Google is still ahead in technical performance, if not in mindshare.