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.