I have GarageBand on both my iPad Air and iPhone 6 (iOS 9.2.1, GarageBand 2.1). I have experienced this problem on both platforms.
I have put many hours recording guitar and bass on relatively small projects (max 10 tracks, about 1 minute). I experienced the "Optimizing Performance" message some times, that lasts for about 5-10 seconds. When I saw this, I thought it was okay; it was caching and/or preprocessing stuff to accelerate 'live' playback. I would accept that.
However, I noticed that the playback sound quality degraded after this. By degradation, I mean the following. I record many high-pitched rock guitar parts with enough gain to have a 'rock' distortion, so the signals have a fair amount of high frequencies. After the "Optimizing Performance" degradation, the high frequencies distorted to be painful to the ears. I am not an expert, but my guess is that the "Optimizing Performance" is in fact a 'downsampling' or a 'compression' of the signals.
Again, I would accept that, if it was only for the 'live playback'; when exporting the project to a sound file (*.m4a), I would expect GarageBand to take the original full bandwidth good quality signals that I recorded in the first place. However, it does not; the resulting exported songs' quality is still bad.
So, my conclusion (that I turned into a question to this community for a confirmation) is that this "Optimizing Performance" is a downsampling/compression of the signals in an 'irreversible' way, which is a pretty way to say that it is ruining everything (by everything, I mean a lot of time and efforts). Again (I am a good guy), that would be okay, but if at least I could be warned that this might soon happen, so that I can clean up my project to release memory. For now it just explodes in my face and I cry.
Anyone else experienced a degradation from the "Optimizing Performance"? Is it really an irreversible downsampling/compression of the signals? Is there a way to block or at least revert this to get back the original signals when it happens?
I have to say that I did not test the 'undo' option; each time it happened, I freaked out and closed/opened back GarageBand (it clears any possible undo). This was panic acting. But still, I would like to hear anyone's experience on this.
Best Answer
Apple is pretty mum on what "Optimizing performance" means, so we only have speculation to go by, but I have noticed a few things and have an idea for you. I have noticed that turning off all reverb and delay effects causes it to happen much less frequently. This would indicate it's some sort of preprocessing or pre-generation to save CPU cycles on playback. The fact that I currently have a 40-minute piece in iOS GarageBand that is taking up 6GB on disk, even though the piece is almost entirely midi tracks and contains no live instrument or vocal tracks, would corroborate that it is storing audio tracks. This is also corroborated by the fact that if I change the tempo, it reliably immediately does an "Optimizing Performance" the very next time I try to play it.
Remember, GarageBand is much more processor-intensive than ordinary audio, because instead of just a left and right channel, it has individual left-and-right channels for every track, which must individually be panned, processed with reverb/delay, etc. So it's entirely possible that it's generating much lower-bandwidth lossy-compression versions of your audio for playback within Mobile GarageBand just to reduce processor load while multiple tracks are playing.
However, based solely on long experience, I would conjecture that it does not reduce the quality of your original audio, but rather, generates a second "optimized" file for use just within the program during playback. I bet if you were to export the entire project to an audio file, your original guitar sound wound be there in sparkling full fidelity.
If you wanted to test this out and report the results back here, I bet you'd be doing a service to the community and the odd Googler who finds this while hunting down the same question.
Obviously, if I'm right about that, it would mean iOS GarageBand is not an ideal app to do final mixes on, since you can't actually hear what it will really sound like until you export the results. Hmmm... An Apple app that includes a fundamental shortcoming which makes it a less-than-ideal tool for a serious user who wants it to do what they're advertising it for... Could that even be possible? Hmmmm.