How much engineering effort is spent on optimization and improving performance, as opposed to changing the look'n'feel and adding consumer-focused features?
Apple works hard on power efficiency. You don't get high battery life for free. Memory usage may get less attention, but, for example, they do 'swap' to memory by compressing pages (does it help? I wouldn't know. Read http://dfrws.org/2014/proceedings/DFRWS2014-1.pdf)
Edit: oops. That paper isn't evaluating performance, as I thought it would.
How much memory does Mac OS X Yosemite need?
How much engineering effort is spent on optimization and improving performance, as opposed to changing the look'n'feel and adding consumer-focused features?