Just playing around with these settings when compiling msp externals and thought the results might be interesting to others.
The UB SDK sets the optimisation level to Os (Fastest / Smallest) for deployment (rather than O3 - fastest - which apple say may produce larger binaries).
I discovered the following:
1 - The O3 Faster setting may produce code that runs quite a lot faster. As an example I compiled the plus~ project in the SDK with this setting and also with the Os (Fastest/Smallest) setting. I then tested a patch filled only with plus~ objects (both inlets connected to a signal)
Under Os - CPU was at 56-58%
Under O3 - CPU was at 39-42%
Which I think is a pretty decent saving. In both cases the external came out as being 36K.
I also had the same result with the code for times~ from an older SDK (also
2 - On the other hand...
A couple of more complex msp externals were marginally slower with this setting (and a bit bigger as well)
These observations hold true for intel only (Compiled and tested on a MBP only).
So it looks like it might be trying different settings for each individual case.