Can I ask you for some clarification about this question?
The first thing comes to my mind is whether the format of recorded speech data in Objective-C affects the result.
Are you asking about the format of your acoustic model recording or the input format on the iPhone? The input format of speech is always PCM and it is only ever stored in memory in a ringbuffer, so there is no format question there or any reading/writing to or from disk, it’s just the raw samples that are streamed from the mic being read out of a C buffer. It’s about as low-latency as it can be without requiring so many callbacks per second that it would hurt performance.