My big pet peeve is that many mobile apps don't keep any local data; every time you load up the app is like an ST:TNG episode reset, as if nothing happened before. Save the data locally, serve that to me when I load up the app, and only once you've done that, check out the backend to see if the data needs to be refreshed. Even though most apps have many clients/endpoints nowadays, most users still access them from only one end-point, so load data locally first makes things much faster. The same can be done with web apps and localStorage.