When testing your app, keep a keen eye out for how and when progress indicators are showing up, or how they aren’t showing up for that matter. Generally speaking progress indicators show that some long-running process is happening or that the app is waiting for some data or other results to be delievered. The latter is probably the most common, because long running processes like image manipulation are under the app’s control and is just a matter of crunching the pixels. Network latency, on the other hand, whether from connection timeouts of slow transfer speeds, isn’t something the app can do much about other than wait.
To simulate a slow connection, you can, of course, create additional traffic on your connection while you’re testing the app–e.g. playing videos, running big downloads (and uploads), and so forth. The faster your connection, the more you’ll have to load it up. You might also be able to change settings in your modem or router to make everything run more slowly. For example, if you have a wireless-N router, change it to run only at wireless-A speeds, then load it up with extra work.
Whatever the case, slowing down the network speed will help show where and how progress indicators are being used. You’ll want to especially look for places where your app just appears to sit without any visual indication of the work it’s doing. This reveals that your code is making assumptions about how quickly data is coming back and that you’ll need to put up a progress indicator if the user waits for more than 1-2 seconds (the recommended timeout for showing an indicator).
On a slow connection, too, you can better evaluate whether you’re overusing progress indicators. That is, while it’s nice that we have controls to show work happening, they are something that users will get tired of looking at over and over. So think about how you might architect the app to wholly eliminate the need for those indicators. For example, if you are switching between a page with a list of items and a details page for one item, and there’s a delay in that switch, it’s a good place to simply switch the visibility of two pages that are fully constructed, rather than doing a real navigation that would tear down and rebuild the pages each time. The user won’t be able to tell the difference, except that everything is running much faster. Indeed, building a page with a list control of hundreds or thousands of items is a very expensive process, so avoiding that work is a good thing, especially when the list page itself doesn’t change in the process of navigating in and out of details.
On the flip side, also test your app on the fastest connection you can find and see if there are any places where progress indicators show up unnecessarily. This would also reveal places where you’re assuming that the data will take a long time to obtain, and where a progress indicator might just flash on the screen momentarily and create visual noise. In those cases you’ll want to make sure you again use a timeout before the indicator appears.