For instance, doing single calls to APIs for multiple pieces of information instead of a bunch of calls. If you design for these up front, you can then head the problem off before it even becomes a problem. It's not premature because users expect it. They do notice network lag, they do notice battery usage, and they will give you one star on the app store or play store, etc. I've literally seen this play out in real life. It's no different than making sure your website loads quickly IMHO. You still have to do all those other things you listed to worry about, but this isn't to be glossed over and addressed when you realize people are noticing.